XML 18 R7.htm IDEA: XBRL DOCUMENT v3.22.2.2
Consolidated Schedule of Investments
€ in Thousands, £ in Thousands, kr in Thousands, kr in Thousands, $ in Thousands, $ in Thousands, $ in Thousands
 9 Months Ended 12 Months Ended
Sep. 30, 2022
USD ($)
Dec. 31, 2021
USD ($)
Sep. 30, 2022
AUD ($)
Sep. 30, 2022
EUR (€)
Sep. 30, 2022
GBP (£)
Sep. 30, 2022
CAD ($)
Dec. 31, 2021
AUD ($)
Dec. 31, 2021
EUR (€)
Dec. 31, 2021
GBP (£)
Dec. 31, 2021
CAD ($)
Dec. 31, 2021
DKK (kr)
Dec. 31, 2021
NOK (kr)
Consolidated Schedule of Investments                        
Cost $ 2,356,143 $ 2,311,384                    
Market Value $ 2,293,522 $ 2,289,105                    
Aerospace and Defense Sector [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 287,223 $ 109,341                    
Market Value $ 105,303 $ 90,029                    
% of NAV 25.10% 8.20% 25.10% 25.10% 25.10% 25.10% 8.20% 8.20% 8.20% 8.20% 8.20% 8.20%
Automotive Sector [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 78,189 $ 87,597                    
Market Value $ 78,215 $ 88,555                    
% of NAV 7.10% 8.10% 7.10% 7.10% 7.10% 7.10% 8.10% 8.10% 8.10% 8.10% 8.10% 8.10%
Banking Finance Insurance And Real Estate Sector [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Cost $ 150                      
Market Value $ 160                      
% of NAV 0.00%   0.00% 0.00% 0.00% 0.00%            
Banking [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Cost $ 7,736 $ 18,370                    
Market Value $ 7,980 $ 18,690                    
% of NAV 0.70% 1.70% 0.70% 0.70% 0.70% 0.70% 1.70% 1.70% 1.70% 1.70% 1.70% 1.70%
Beverage Foodand Tobacco Sector [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 639 $ 843                    
Market Value $ 121 $ 228                    
% of NAV 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Cost $ 6,720 $ 6,720                    
Market Value $ 14,581 $ 19,527                    
% of NAV 1.30% 1.80% 1.30% 1.30% 1.30% 1.30% 1.80% 1.80% 1.80% 1.80% 1.80% 1.80%
Capital Equipment Sector [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 31,046 $ 65,129                    
Market Value $ 30,728 $ 64,841                    
% of NAV 2.80% 5.90% 2.80% 2.80% 2.80% 2.80% 5.90% 5.90% 5.90% 5.90% 5.90% 5.90%
Chemicals, Plastics and Rubber [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 14,177 $ 26,135                    
Market Value $ 14,065 $ 26,863                    
% of NAV 1.30% 2.40% 1.30% 1.30% 1.30% 1.30% 2.40% 2.40% 2.40% 2.40% 2.40% 2.40%
Construction Sector [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 117,017 $ 70,256                    
Market Value $ 113,634 $ 68,570                    
% of NAV 10.40% 6.20% 10.40% 10.40% 10.40% 10.40% 6.20% 6.20% 6.20% 6.20% 6.20% 6.20%
Consumer Goods Durable Sector [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 84,266 $ 83,903                    
Market Value $ 74,692 $ 76,575                    
% of NAV 6.80% 7.00% 6.80% 6.80% 6.80% 6.80% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Consumer Goods Non Durable Sector [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 130,849 $ 179,733                    
Market Value $ 129,695 $ 182,063                    
% of NAV 11.90% 16.60% 11.90% 11.90% 11.90% 11.90% 16.60% 16.60% 16.60% 16.60% 16.60% 16.60%
Consumer Goods Wholesale Sector [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 8,839 $ 14,870                    
Market Value $ 7,086 $ 14,827                    
% of NAV 0.60% 1.30% 0.60% 0.60% 0.60% 0.60% 1.30% 1.30% 1.30% 1.30% 1.30% 1.30%
Containers Packaging And Glass [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 16,372 $ 27,379                    
Market Value $ 16,071 $ 27,378                    
% of NAV 1.50% 2.50% 1.50% 1.50% 1.50% 1.50% 2.50% 2.50% 2.50% 2.50% 2.50% 2.50%
Energy Oil And Gas [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 36,231 $ 47,148                    
Market Value $ 36,427 $ 47,492                    
% of NAV 3.30% 4.30% 3.30% 3.30% 3.30% 3.30% 4.30% 4.30% 4.30% 4.30% 4.30% 4.30%
Energy Oil And Gas Noncontrolled Affiliate Investments [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Cost $ 20,771 $ 22,440                    
Market Value $ 38,471 $ 32,056                    
% of NAV 3.50% 2.90% 3.50% 3.50% 3.50% 3.50% 2.90% 2.90% 2.90% 2.90% 2.90% 2.90%
Energy Sector [Member]                        
Consolidated Schedule of Investments                        
Cost $ 57,002 $ 69,588                    
Market Value $ 74,898 $ 79,548                    
Environmental Industries Sector [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Cost $ 35,140                      
Market Value $ 34,538                      
% of NAV 3.20%   3.20% 3.20% 3.20% 3.20%            
Fire Finance [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 49,741 $ 24,245                    
Market Value $ 47,545 $ 24,452                    
% of NAV 2.30% 2.20% 2.30% 2.30% 2.30% 2.30% 2.20% 2.20% 2.20% 2.20% 2.20% 2.20%
Fire Finance Non Controlled Affiliate                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member]   Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]            
Cost $ 24,051                      
Market Value $ 22,298                      
% of NAV 2.00%   2.00% 2.00% 2.00% 2.00%            
Fire Insurance [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 64,944 $ 62,165                    
Market Value $ 62,954 $ 63,123                    
% of NAV 5.70% 5.70% 5.70% 5.70% 5.70% 5.70% 5.70% 5.70% 5.70% 5.70% 5.70% 5.70%
Healthcare And Pharmaceuticals [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 101,715 $ 102,116                    
Market Value $ 99,633 $ 100,771                    
% of NAV 9.10% 9.20% 9.10% 9.10% 9.10% 9.10% 9.20% 9.20% 9.20% 9.20% 9.20% 9.20%
High Tech Industries [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 220,512 $ 182,811                    
Market Value $ 217,497 $ 183,069                    
% of NAV 19.80% 16.60% 19.80% 19.80% 19.80% 19.80% 16.60% 16.60% 16.60% 16.60% 16.60% 16.60%
Hospitality Holdings [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 5,000 $ 5,000                    
Market Value $ 5,841 $ 5,442                    
% of NAV 0.50% 0.50% 0.50% 0.50% 0.50% 0.50% 0.50% 0.50% 0.50% 0.50% 0.50% 0.50%
Hotel, Gaming And Leisure [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 37,159 $ 53,232                    
Market Value $ 36,068 $ 51,956                    
% of NAV 3.30% 4.70% 3.30% 3.30% 3.30% 3.30% 4.70% 4.70% 4.70% 4.70% 4.70% 4.70%
Investment Vehicles [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]
Cost $ 284,780 $ 165,033                    
Market Value $ 282,498 $ 169,881                    
% of NAV 25.80% 15.40% 25.80% 25.80% 25.80% 25.80% 15.40% 15.40% 15.40% 15.40% 15.40% 15.40%
Media: Advertising, Printing & Publishing                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 54,169 $ 50,389                    
Market Value $ 27,819 $ 39,459                    
% of NAV 2.50% 3.60% 2.50% 2.50% 2.50% 2.50% 3.60% 3.60% 3.60% 3.60% 3.60% 3.60%
Media: Broadcasting & Subscription                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 2,825 $ 8,740                    
Market Value $ 2,717 $ 8,979                    
% of NAV 0.20% 0.80% 0.20% 0.20% 0.20% 0.20% 0.80% 0.80% 0.80% 0.80% 0.80% 0.80%
Media: Diversified & Production                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 36,721 $ 46,420                    
Market Value $ 34,136 $ 44,835                    
% of NAV 3.10% 4.10% 3.10% 3.10% 3.10% 3.10% 4.10% 4.10% 4.10% 4.10% 4.10% 4.10%
Media Publishing [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Cost $ 21,507                      
Market Value $ 20,356                      
% of NAV 1.90%   1.90% 1.90% 1.90% 1.90%            
Retail [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 60,350 $ 83,036                    
Market Value $ 53,266 $ 83,064                    
% of NAV 4.90% 7.60% 4.90% 4.90% 4.90% 4.90% 7.60% 7.60% 7.60% 7.60% 7.60% 7.60%
Services, Business [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 170,902 $ 226,035                    
Market Value $ 163,918 $ 225,437                    
% of NAV 15.00% 20.50% 15.00% 15.00% 15.00% 15.00% 20.50% 20.50% 20.50% 20.50% 20.50% 20.50%
Services, Consumer [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 17,585 $ 40,467                    
Market Value $ 17,823 $ 41,236                    
% of NAV 1.60% 3.70% 1.60% 1.60% 1.60% 1.60% 3.70% 3.70% 3.70% 3.70% 3.70% 3.70%
Telecommunications [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 46,415 $ 40,938                    
Market Value $ 47,204 $ 41,570                    
% of NAV 4.30% 3.80% 4.30% 4.30% 4.30% 4.30% 3.80% 3.80% 3.80% 3.80% 3.80% 3.80%
Transportation Cargo [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 63,187 $ 88,860                    
Market Value $ 66,773 $ 91,607                    
% of NAV 6.10% 8.30% 6.10% 6.10% 6.10% 6.10% 8.30% 8.30% 8.30% 8.30% 8.30% 8.30%
Transportation Cargo Controlled Affiliate [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]
Cost $ 23,665 $ 14,152                    
Market Value $ 25,101 $ 14,851                    
% of NAV 2.30% 1.40% 2.30% 2.30% 2.30% 2.30% 1.40% 1.40% 1.40% 1.40% 1.40% 1.40%
Transportation Consumer [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Cost $ 8,663                      
Market Value $ 6,358                      
% of NAV 0.60%   0.60% 0.60% 0.60% 0.60%            
Transportation Consumer Non Controlled Non Affiliate [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost   $ 8,299                    
Market Value   $ 7,333                    
% of NAV   0.70%         0.70% 0.70% 0.70% 0.70% 0.70% 0.70%
Transportation Consumer Non Controlled Affiliate [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Cost $ 73,070 $ 71,728                    
Market Value $ 71,793 $ 61,707                    
% of NAV 6.60% 5.60% 6.60% 6.60% 6.60% 6.60% 5.60% 5.60% 5.60% 5.60% 5.60% 5.60%
Wholesale [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 26,426 $ 77,737                    
Market Value $ 27,640 $ 80,070                    
% of NAV 2.50% 7.30% 2.50% 2.50% 2.50% 2.50% 7.30% 7.30% 7.30% 7.30% 7.30% 7.30%
Cash Equivalents [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member]
Cost $ 31,249 $ 177,554                    
Market Value $ 31,249 $ 177,554                    
% of NAV 2.90% 16.10% 2.90% 2.90% 2.90% 2.90% 16.10% 16.10% 16.10% 16.10% 16.10% 16.10%
Common Stock [Member]                        
Consolidated Schedule of Investments                        
Market Value $ 4,100                      
Equity Interest Investment Vehicles [Member]                        
Consolidated Schedule of Investments                        
Cost 60,486 $ 39,596                    
Market Value 58,647 44,444                    
Preferred Stock [Member]                        
Consolidated Schedule of Investments                        
Cost 50,990 42,452                    
Market Value 71,738 53,991                    
Second Lien Senior Secured Loan [Member]                        
Consolidated Schedule of Investments                        
Market Value 96,054 118,561                    
Subordinated Debt                        
Consolidated Schedule of Investments                        
Cost 38,734 19,635                    
Market Value 39,478 20,027                    
Warrant [Member]                        
Consolidated Schedule of Investments                        
Cost 480 2                    
Market Value 530 126                    
Investment Holdings [Member]                        
Consolidated Schedule of Investments                        
Cost 2,356,143 2,311,384                    
Market Value $ 2,293,522 $ 2,289,105                    
% of NAV 209.20% 208.10% 209.20% 209.20% 209.20% 209.20% 208.10% 208.10% 208.10% 208.10% 208.10% 208.10%
Investment Holdings and Cash Equivalents [Member]                        
Consolidated Schedule of Investments                        
Cost $ 2,387,392 $ 2,488,938                    
Market Value $ 2,324,771 $ 2,466,659                    
% of NAV 212.10% 224.20% 212.10% 212.10% 212.10% 212.10% 224.20% 224.20% 224.20% 224.20% 224.20% 224.20%
Investment, Affiliated Issuer, Controlled [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member]   Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]            
Cost $ 419,887 $ 288,526                    
Market Value $ 412,902 $ 274,761                    
% of NAV 37.70% 25.00% 37.70% 37.70% 37.70% 37.70% 25.00% 25.00% 25.00% 25.00% 25.00% 25.00%
Investment, Affiliated Issuer, Controlled [Member] | Aerospace and Defense Sector [Member]                        
Consolidated Schedule of Investments                        
Cost $ 111,442                      
% of NAV 9.60%   9.60% 9.60% 9.60% 9.60%            
Investment, Affiliated Issuer, Noncontrolled [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Cost $ 124,612 $ 100,888                    
Market Value $ 147,143 $ 113,290                    
% of NAV 13.40% 10.30% 13.40% 13.40% 13.40% 13.40% 10.30% 10.30% 10.30% 10.30% 10.30% 10.30%
InvestmentNonaffiliatedIssuerNoncontrolledMember | Aerospace and Defense Sector [Member]                        
Consolidated Schedule of Investments                        
Cost   $ 200,117                    
Market Value $ 274,815 $ 192,569                    
% of NAV   17.50%         17.50% 17.50% 17.50% 17.50% 17.50% 17.50%
InvestmentNonaffiliatedIssuerNoncontrolledMember | Fire Finance [Member]                        
Consolidated Schedule of Investments                        
Cost 25,690                      
Market Value $ 25,247                      
Investment, Unaffiliated Issuer [Member]                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Cost $ 1,811,644 $ 1,921,970                    
Market Value $ 1,733,477 $ 1,901,054                    
% of NAV 158.10% 172.80% 158.10% 158.10% 158.10% 158.10% 172.80% 172.80% 172.80% 172.80% 172.80% 172.80%
Investment, Identifier [Axis]: 9 Story Media Group Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Nine Story Media Group Inc [Member] Nine Story Media Group Inc [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production
Maturity date Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026
Cost $ (1)                      
Investment, Interest Rate, Floor   0.75%         0.75% 0.75% 0.75% 0.75% 0.75% 0.75%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: 9 Story Media Group Inc. First Lien Senior Secured Loan Cdor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Nine Story Media Group Inc [Member] Nine Story Media Group Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Media: Diversified & Production   Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production            
Spread Above Index (as a percent) 5.25% 5.50% 5.25% 5.25% 5.25% 5.25% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 9.03% 6.25% 9.03% 9.03% 9.03% 9.03% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026
Principal/Shares           $ 1,295       $ 72    
Cost $ 1,004                 54    
Market Value $ 937                 $ 57    
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember
Investment, Interest Rate, Floor 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: 9 Story Media Group Inc. First Lien Senior Secured Loan Euibor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Nine Story Media Group Inc [Member] Nine Story Media Group Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production
Spread Above Index (as a percent) 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25%
Interest rate 5.83% 5.25% 5.83% 5.83% 5.83% 5.83% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25%
Maturity date Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026
Principal/Shares | €       € 586       € 39        
Cost $ 621             45        
Market Value $ 573             € 44        
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: A&R Logistics, Inc First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ARLogisticsIncMember bcsf:ARLogisticsIncMember                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Spread Above Index (as a percent) 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate 8.35% 7.00% 8.35% 8.35% 8.35% 8.35% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025
Principal/Shares $ 2,066 $ 2,815                    
Cost 1,954 2,748                    
Market Value $ 2,066 $ 2,815                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: A&R Logistics, Inc First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ARLogisticsIncMember bcsf:ARLogisticsIncMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Spread Above Index (as a percent) 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate 8.23% 7.00% 8.23% 8.23% 8.23% 8.23% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025
Principal/Shares $ 5,928 $ 43,092                    
Cost 5,881 42,527                    
Market Value $ 5,928 $ 43,092                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: A&R Logistics, Inc First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ARLogisticsIncMember bcsf:ARLogisticsIncMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Spread Above Index (as a percent) 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate 8.23% 7.00% 8.23% 8.23% 8.23% 8.23% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025
Principal/Shares $ 2,405 $ 2,423                    
Cost 2,379 2,391                    
Market Value $ 2,405 $ 2,423                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: A&R Logistics, Inc First Lien Senior Secured Loan 3                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ARLogisticsIncMember bcsf:ARLogisticsIncMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Spread Above Index (as a percent) 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate 9.71% 7.00% 9.71% 9.71% 9.71% 9.71% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025
Principal/Shares $ 32,064 $ 5,974                    
Cost 31,731 5,916                    
Market Value $ 32,064 $ 5,974                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: A&R Logistics, Inc First Lien Senior Secured Loan 4                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ARLogisticsIncMember bcsf:ARLogisticsIncMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Spread Above Index (as a percent) 6.50% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Interest rate 8.73% 7.50% 8.73% 8.73% 8.73% 8.73% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Maturity date May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025
Principal/Shares $ 2,695 $ 2,716                    
Cost 2,679 2,695                    
Market Value $ 2,695 $ 2,716                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: A&R Logistics, Inc First Lien Senior Secured Loan 5                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ARLogisticsIncMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member]   Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]            
Spread Above Index (as a percent) 6.60%   6.60% 6.60% 6.60% 6.60%            
Interest rate 9.23%   9.23% 9.23% 9.23% 9.23%            
Maturity date May 05, 2025   May 05, 2025 May 05, 2025 May 05, 2025 May 05, 2025            
Principal/Shares $ 1,319                      
Cost 1,306                      
Market Value $ 1,319                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: ACAMS Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ACAMSMember                      
Investment, Type [Extensible Enumeration] Common Stock [Member]   Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Principal/Shares $ 3,337                      
Cost 3,337                      
Market Value $ 3,309                      
Acquisition date Mar. 10, 2022                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: ACM dcBLOX LLC Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:ACMdcBLOXLLCMember                    
Investment, Type [Extensible Enumeration]   Preferred Stock [Member]         Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration]   Telecommunications [Member]         Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]
Principal/Shares   $ 3,822                    
Cost   3,851                    
Market Value   $ 4,130                    
Acquisition date   Mar. 22, 2021                    
Investment, Restriction Status [true false]   true         true true true true true true
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   true         true true true true true true
Investment, Identifier [Axis]: ADT Pizza, LLC Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ADTPizzaLLCMember bcsf:ADTPizzaLLCMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Equity Securities [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member]
Investment, Industry Sector [Extensible Enumeration] Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member] Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member] Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member] Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member] Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member] Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member] Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member] Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member] Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member] Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member] Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member] Beverage Food And Tobacco Sector Non-controlled Affiliate Investments [Member]
Principal/Shares $ 6,720 $ 6,720                    
Cost 6,720 6,720                    
Market Value $ 14,581 $ 19,527                    
Acquisition date Oct. 29, 2018 Oct. 29, 2018                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: AMCP Clean Acquisition Company, LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:AMCPCleanAcquisitionCompanyLLCMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 4.25%   4.25% 4.25% 4.25% 4.25%            
Interest rate 7.37%   7.37% 7.37% 7.37% 7.37%            
Maturity date Jul. 10, 2025   Jul. 10, 2025 Jul. 10, 2025 Jul. 10, 2025 Jul. 10, 2025            
Principal/Shares $ 16,297                      
Cost 16,174                      
Market Value $ 13,222                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: AMCP Clean Acquisition Company, LLC First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:AMCPCleanAcquisitionCompanyLLCMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 4.25%   4.25% 4.25% 4.25% 4.25%            
Interest rate 7.37%   7.37% 7.37% 7.37% 7.37%            
Maturity date Jul. 10, 2025   Jul. 10, 2025 Jul. 10, 2025 Jul. 10, 2025 Jul. 10, 2025            
Principal/Shares $ 3,944                      
Cost 3,914                      
Market Value $ 3,198                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: AMCP Clean Acquisition Company, LLC First Lien Senior Secured Loan - Delayed Draw Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:AMCPCleanAcquisitionCompanyLLCMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   4.25%         4.25% 4.25% 4.25% 4.25% 4.25% 4.25%
Interest rate   4.35%         4.35% 4.35% 4.35% 4.35% 4.35% 4.35%
Maturity date   Jul. 10, 2025         Jul. 10, 2025 Jul. 10, 2025 Jul. 10, 2025 Jul. 10, 2025 Jul. 10, 2025 Jul. 10, 2025
Principal/Shares   $ 3,816                    
Cost   3,810                    
Market Value   $ 3,189                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: AMCP Clean Acquisition Company, LLC First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:AMCPCleanAcquisitionCompanyLLCMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   4.25%         4.25% 4.25% 4.25% 4.25% 4.25% 4.25%
Interest rate   4.35%         4.35% 4.35% 4.35% 4.35% 4.35% 4.35%
Maturity date   Jul. 10, 2025         Jul. 10, 2025 Jul. 10, 2025 Jul. 10, 2025 Jul. 10, 2025 Jul. 10, 2025 Jul. 10, 2025
Principal/Shares   $ 15,767                    
Cost   15,747                    
Market Value   $ 13,176                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: AMI US Holdings Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:AMIUSHoldingsIncMember bcsf:AMIUSHoldingsIncMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 5.25% 5.50% 5.25% 5.25% 5.25% 5.25% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 7.81% 6.50% 7.81% 7.81% 7.81% 7.81% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Apr. 01, 2025 Apr. 01, 2025 Apr. 01, 2025 Apr. 01, 2025 Apr. 01, 2025 Apr. 01, 2025 Apr. 01, 2025 Apr. 01, 2025 Apr. 01, 2025 Apr. 01, 2025 Apr. 01, 2025 Apr. 01, 2025
Principal/Shares $ 3,866 $ 12,892                    
Cost 3,830 12,735                    
Market Value $ 3,866 $ 12,892                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: AMI US Holdings Inc. First Lien Senior Secured Loan - Revolver Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:AMIUSHoldingsIncMember                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   High Tech Industries [Member]         High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent)   5.25%         5.25% 5.25% 5.25% 5.25% 5.25% 5.25%
Interest rate   5.35%         5.35% 5.35% 5.35% 5.35% 5.35% 5.35%
Maturity date   Apr. 01, 2024         Apr. 01, 2024 Apr. 01, 2024 Apr. 01, 2024 Apr. 01, 2024 Apr. 01, 2024 Apr. 01, 2024
Principal/Shares   $ 698                    
Cost   682                    
Market Value   $ 698                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   0.00%         0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: AQ Software Corporation Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:AQSoftwareCorporationMember bcsf:AQSoftwareCorporationMember                    
Investment, Type [Extensible Enumeration] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Principal/Shares $ 1 $ 1                    
Cost 1,107 1,029                    
Market Value $ 1,104 $ 1,029                    
Acquisition date Dec. 10, 2021                      
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] true false true true true true false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true false true true true true false false false false false false
Investment, Identifier [Axis]: AQ Software Corporation Preferred Equity 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:AQSoftwareCorporationMember bcsf:AQSoftwareCorporationMember                    
Investment, Type [Extensible Enumeration] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Principal/Shares $ 2 $ 2                    
Cost 1,844 1,715                    
Market Value $ 1,839 $ 1,715                    
Acquisition date Apr. 14, 2022 Dec. 10, 2021                    
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] true false true true true true false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true false true true true true false false false false false false
Investment, Identifier [Axis]: AQ Software Corporation Preferred Equity 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:AQSoftwareCorporationMember                      
Investment, Type [Extensible Enumeration] Preferred Stock [Member]   Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Principal/Shares $ 1                      
Cost 507                      
Market Value $ 506                      
Acquisition date   Dec. 10, 2021                    
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: ARL Holdings, LLC Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:ARLHoldingsLLCMember                    
Investment, Type [Extensible Enumeration]   Common Stock [Member]         Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration]   Transportation Cargo [Member]         Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Principal/Shares   $ 445                    
Cost   445                    
Market Value   $ 575                    
Acquisition date   May 03, 2019                    
Investment, Restriction Status [true false]   true         true true true true true true
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   true         true true true true true true
Investment, Identifier [Axis]: ARL Holdings, LLC Equity Interest 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ARLHoldingsLLCMember bcsf:ARLHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Principal/Shares $ 0 $ 9                    
Cost 445 9                    
Market Value $ 619 $ 81                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: ARL Holdings, LLC Equity Interest 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ARLHoldingsLLCMember                      
Investment, Type [Extensible Enumeration] Common Stock [Member]   Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member]   Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]            
Principal/Shares $ 9                      
Cost 9                      
Market Value $ 334                      
Acquisition date May 03, 2019 May 03, 2019                    
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: ASP-r-pac Acquisition Co LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:ASP-r-pacAcquisitionCoLLCMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Containers Packaging And Glass [Member]         Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member]
Spread Above Index (as a percent)   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date   Dec. 29, 2027         Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027
Principal/Shares   $ 27,339                    
Cost   26,793                    
Market Value   $ 26,792                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   0.75%         0.75% 0.75% 0.75% 0.75% 0.75% 0.75%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: ASP-r-pac Acquisition Co LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ASP-r-pacAcquisitionCoLLCMember bcsf:ASP-r-pacAcquisitionCoLLCMember                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member]
Spread Above Index (as a percent)   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027
Principal/Shares   $ 651                    
Cost   586                    
Cost $ (57)                      
Market Value $ (24) $ 586                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   0.75%         0.75% 0.75% 0.75% 0.75% 0.75% 0.75%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: ASP-r-pac Acquisition Co LLC First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ASP-r-pacAcquisitionCoLLCMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Containers Packaging And Glass [Member]   Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member]            
Spread Above Index (as a percent) 6.00%   6.00% 6.00% 6.00% 6.00%            
Interest rate 9.67%   9.67% 9.67% 9.67% 9.67%            
Maturity date Dec. 29, 2027   Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027 Dec. 29, 2027            
Principal/Shares $ 4,093                      
Cost 4,041                      
Market Value $ 4,063                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Abracon Group Holding, LLC Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:AbraconGroupHoldingLLCMember                    
Investment, Type [Extensible Enumeration]   Equity Securities [Member]         Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member]
Investment, Industry Sector [Extensible Enumeration]   Wholesale [Member]         Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member]
Principal/Shares   $ 2                    
Cost   1,833                    
Market Value   $ 3,282                    
Acquisition date   Jul. 18, 2018                    
Investment, Restriction Status [true false]   true         true true true true true true
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   true         true true true true true true
Investment, Identifier [Axis]: Abracon Group Holding, LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:AbraconGroupHoldingLLCMember bcsf:AbraconGroupHoldingLLCMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member]
Spread Above Index (as a percent) 6.00% 5.25% 6.00% 6.00% 6.00% 6.00% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25%
Interest rate 7.86% 6.25% 7.86% 7.86% 7.86% 7.86% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date Jul. 06, 2028 Jul. 18, 2024 Jul. 06, 2028 Jul. 06, 2028 Jul. 06, 2028 Jul. 06, 2028 Jul. 18, 2024 Jul. 18, 2024 Jul. 18, 2024 Jul. 18, 2024 Jul. 18, 2024 Jul. 18, 2024
Principal/Shares $ 11,547 $ 35,363                    
Cost 11,322 35,270                    
Market Value $ 11,315 $ 35,363                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Abracon Group Holding, LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:AbraconGroupHoldingLLCMember bcsf:AbraconGroupHoldingLLCMember                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member]
Maturity date Jul. 06, 2028 Jul. 18, 2024 Jul. 06, 2028 Jul. 06, 2028 Jul. 06, 2028 Jul. 06, 2028 Jul. 18, 2024 Jul. 18, 2024 Jul. 18, 2024 Jul. 18, 2024 Jul. 18, 2024 Jul. 18, 2024
Cost $ (39) $ (18)                    
Market Value $ (40)                      
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Abracon Group Holding, LLC. First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:AbraconGroupHoldingLLCMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Wholesale [Member]   Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member]            
Maturity date Jul. 06, 2028   Jul. 06, 2028 Jul. 06, 2028 Jul. 06, 2028 Jul. 06, 2028            
Cost $ (49)                      
Market Value $ (50)                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Access First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Access [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 5.75%   5.75% 5.75% 5.75% 5.75%            
Maturity date Jun. 02, 2029   Jun. 02, 2029 Jun. 02, 2029 Jun. 02, 2029 Jun. 02, 2029            
Principal/Shares | £         £ 3,217              
Cost $ 3,318                      
Market Value $ 3,483                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember   bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Access First Lien Senior Secured Loan Sonia                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Access [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 7.44%   7.44% 7.44% 7.44% 7.44%            
Maturity date Jun. 02, 2029   Jun. 02, 2029 Jun. 02, 2029 Jun. 02, 2029 Jun. 02, 2029            
Principal/Shares | £         £ 7,960              
Cost $ 9,770                      
Market Value $ 8,801                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember   bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Aimbridge Acquisition Co., Inc. Second Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:AimbridgeAcquisitionCoIncMember                      
Investment, Type [Extensible Enumeration] Second Lien Senior Secured Loan [Member]   Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Hotel, Gaming And Leisure [Member]   Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member]            
Spread Above Index (as a percent) 7.50%   7.50% 7.50% 7.50% 7.50%            
Interest rate 10.06%   10.06% 10.06% 10.06% 10.06%            
Maturity date Feb. 01, 2027   Feb. 01, 2027 Feb. 01, 2027 Feb. 01, 2027 Feb. 01, 2027            
Principal/Shares $ 14,193                      
Cost 13,906                      
Market Value $ 13,413                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Aimbridge Acquisition Co., Inc. Second Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:AimbridgeAcquisitionCoIncMember                    
Investment, Type [Extensible Enumeration]   Second Lien Senior Secured Loan [Member]         Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Hotel, Gaming And Leisure [Member]         Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member]
Spread Above Index (as a percent)   7.50%         7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Interest rate   7.59%         7.59% 7.59% 7.59% 7.59% 7.59% 7.59%
Maturity date   Feb. 01, 2027         Feb. 01, 2027 Feb. 01, 2027 Feb. 01, 2027 Feb. 01, 2027 Feb. 01, 2027 Feb. 01, 2027
Principal/Shares   $ 20,193                    
Cost   19,772                    
Market Value   $ 18,679                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   0.00%         0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Allworth Financial Group, L.P. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:AllworthFinancialGroupLPMember bcsf:AllworthFinancialGroupLPMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member]
Spread Above Index (as a percent) 4.75% 5.00% 4.75% 4.75% 4.75% 4.75% 5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate 7.88% 6.00% 7.88% 7.88% 7.88% 7.88% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Maturity date Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026
Principal/Shares $ 1,509 $ 10,037                    
Cost 1,493 9,908                    
Market Value $ 1,509 $ 10,037                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Allworth Financial Group, L.P. First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:AllworthFinancialGroupLPMember bcsf:AllworthFinancialGroupLPMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member]
Spread Above Index (as a percent) 4.75% 5.00% 4.75% 4.75% 4.75% 4.75% 5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate 7.88% 6.00% 7.88% 7.88% 7.88% 7.88% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Maturity date Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026
Principal/Shares $ 876 $ 2,528                    
Cost 863 2,476                    
Market Value $ 876 $ 2,528                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Allworth Financial Group, L.P. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:AllworthFinancialGroupLPMember bcsf:AllworthFinancialGroupLPMember                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member]
Maturity date Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026
Cost $ (13) $ (15)                    
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: American Trailer Rental Group Subordinated Debt                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:AmericanTrailerRentalGroupMember bcsf:AmericanTrailerRentalGroupMember                    
Investment, Type [Extensible Enumeration] Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt
Investment, Industry Sector [Extensible Enumeration] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member]
Spread Above Index (as a percent) 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00%
PIK (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%)
Interest rate 11.00% 11.00% 11.00% 11.00% 11.00% 11.00% 11.00% 11.00% 11.00% 11.00% 11.00% 11.00%
Maturity date Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027
Principal/Shares $ 4,973 $ 4,913                    
Cost 4,909 4,842                    
Market Value $ 4,973 $ 4,913                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: American Trailer Rental Group Subordinated Debt 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:AmericanTrailerRentalGroupMember bcsf:AmericanTrailerRentalGroupMember                    
Investment, Type [Extensible Enumeration] Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt
Investment, Industry Sector [Extensible Enumeration] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member]
Spread Above Index (as a percent) 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00%
PIK (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%)
Interest rate 11.00% 11.00% 11.00% 11.00% 11.00% 11.00% 11.00% 11.00% 11.00% 11.00% 11.00% 11.00%
Maturity date Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027
Principal/Shares $ 15,344 $ 15,114                    
Cost 15,053 14,793                    
Market Value $ 15,344 $ 15,114                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: American Trailer Rental Group Subordinated Debt 3                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:AmericanTrailerRentalGroupMember                      
Investment, Type [Extensible Enumeration] Subordinated Debt   Subordinated Debt Subordinated Debt Subordinated Debt Subordinated Debt            
Investment, Industry Sector [Extensible Enumeration] Automotive Sector [Member]   Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member]            
Spread Above Index (as a percent) 9.00%   9.00% 9.00% 9.00% 9.00%            
PIK (2.00%)   (2.00%) (2.00%) (2.00%) (2.00%)            
Interest rate 11.00%   11.00% 11.00% 11.00% 11.00%            
Maturity date Dec. 01, 2027   Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027 Dec. 01, 2027            
Principal/Shares $ 19,161                      
Cost 18,772                      
Market Value $ 19,161                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Amspec Services, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Amspec Services Inc [Member] Amspec Services Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member]
Spread Above Index (as a percent) 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate 9.42% 6.75% 9.42% 9.42% 9.42% 9.42% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024
Principal/Shares $ 2,784 $ 43,207                    
Cost 2,763 42,923                    
Market Value $ 2,784 $ 43,207                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Amspec Services, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Amspec Services Inc [Member]                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Energy Oil And Gas [Member]         Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date   Jul. 02, 2024         Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024
Principal/Shares   $ 1,488                    
Cost   1,457                    
Market Value   $ 1,487                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Amspec Services, Inc. First Lien Senior Secured Loan - Revolver Prime Rate                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Amspec Services Inc [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Energy Oil And Gas [Member]   Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member]            
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 10.00%   10.00% 10.00% 10.00% 10.00%            
Maturity date Jul. 02, 2024   Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024            
Principal/Shares $ 567                      
Cost 546                      
Market Value $ 567                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] us-gaap:PrimeRateMember   us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Amspec Services, Inc. First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Amspec Services Inc [Member] Amspec Services Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member] Energy Oil And Gas [Member]
Spread Above Index (as a percent) 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate 9.42% 6.75% 9.42% 9.42% 9.42% 9.42% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024 Jul. 02, 2024
Principal/Shares $ 33,076 $ 2,798                    
Cost 32,922 2,768                    
Market Value $ 33,076 $ 2,798                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Ansett Aviation Training First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:AnsettAviationTrainingMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member]   Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]            
Spread Above Index (as a percent) 4.69%   4.69% 4.69% 4.69% 4.69%            
Interest rate 5.69%   5.69% 5.69% 5.69% 5.69%            
Maturity date Sep. 24, 2031   Sep. 24, 2031 Sep. 24, 2031 Sep. 24, 2031 Sep. 24, 2031            
Principal/Shares     $ 7,072                  
Cost $ 5,308                      
Market Value $ 4,525                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:BankBillSwapBidRateMember   bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Ansett Aviation Training Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:AnsettAviationTrainingMember                      
Investment, Type [Extensible Enumeration] Common Stock [Member]   Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member]   Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]            
Principal/Shares     $ 5,119                  
Cost     3,842                  
Market Value     $ 3,277                  
Acquisition date Mar. 24, 2022                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: Ansira Holdings, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Ansira Holdings Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Media: Advertising, Printing & Publishing   Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing            
Spread Above Index (as a percent) 6.50%   6.50% 6.50% 6.50% 6.50%            
Interest rate 9.31%   9.31% 9.31% 9.31% 9.31%            
Maturity date Dec. 20, 2024   Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024            
Principal/Shares $ 41,731                      
Cost 40,925                      
Market Value $ 20,866                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Ansira Holdings, Inc. First Lien Senior Secured Loan - Delayed Draw Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Ansira Holdings Inc [Member] Ansira Holdings Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration] Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing
Spread Above Index (as a percent) 6.50% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Interest rate 9.57% 7.50% 9.57% 9.57% 9.57% 9.57% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Maturity date Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024
Principal/Shares $ 5,064 $ 4,873                    
Cost 5,066 4,874                    
Market Value $ 2,532 $ 3,862                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Ansira Holdings, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Ansira Holdings Inc [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Media: Advertising, Printing & Publishing   Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 8.79%   8.79% 8.79% 8.79% 8.79%            
Maturity date Dec. 20, 2024   Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024            
Principal/Shares $ 5,383                      
Cost 5,383                      
Market Value $ 1,842                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Ansira Holdings, Inc. First Lien Senior Secured Loan - Revolver Prime                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Ansira Holdings Inc [Member]                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Media: Advertising, Printing & Publishing         Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   7.41%         7.41% 7.41% 7.41% 7.41% 7.41% 7.41%
Maturity date   Dec. 20, 2024         Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024
Principal/Shares   $ 5,383                    
Cost   5,383                    
Market Value   $ 3,913                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   us-gaap:PrimeRateMember         us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember
Investment, Interest Rate, Floor   2.00%         2.00% 2.00% 2.00% 2.00% 2.00% 2.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Ansira Holdings, Inc. First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Ansira Holdings Inc [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Media: Advertising, Printing & Publishing         Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing
Spread Above Index (as a percent)   6.50%         6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Interest rate   7.50%         7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Maturity date   Dec. 20, 2024         Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024 Dec. 20, 2024
Principal/Shares   $ 40,086                    
Cost   40,057                    
Market Value   $ 31,768                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Ap Plastic Group Llc First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ApPlasticsGroupLlcMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Chemicals, Plastics and Rubber [Member]   Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member]            
Spread Above Index (as a percent) 4.75%   4.75% 4.75% 4.75% 4.75%            
Interest rate 7.87%   7.87% 7.87% 7.87% 7.87%            
Maturity date Aug. 10, 2028   Aug. 10, 2028 Aug. 10, 2028 Aug. 10, 2028 Aug. 10, 2028            
Principal/Shares $ 7,306                      
Cost 7,087                      
Market Value $ 7,014                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Apollo Intelligence Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Apollo Intelligence [Member]                      
Investment, Type [Extensible Enumeration] Common Stock [Member]   Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member]   Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]            
Principal/Shares $ 32                      
Cost 3,162                      
Market Value $ 3,182                      
Acquisition date Jun. 01, 2022                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: Apollo Intelligence First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Apollo Intelligence [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member]   Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 8.26%   8.26% 8.26% 8.26% 8.26%            
Maturity date Jun. 01, 2028   Jun. 01, 2028 Jun. 01, 2028 Jun. 01, 2028 Jun. 01, 2028            
Principal/Shares $ 15,425                      
Cost 15,277                      
Market Value $ 15,271                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Apollo Intelligence First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Apollo Intelligence [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member]   Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]            
Maturity date Jun. 01, 2028   Jun. 01, 2028 Jun. 01, 2028 Jun. 01, 2028 Jun. 01, 2028            
Cost $ (91)                      
Market Value $ (96)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Apollo Intelligence First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Apollo Intelligence [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member]   Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]            
Maturity date Jun. 01, 2028   Jun. 01, 2028 Jun. 01, 2028 Jun. 01, 2028 Jun. 01, 2028            
Cost $ (68)                      
Market Value $ (72)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Applitools First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Applitools [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 9.30%   9.30% 9.30% 9.30% 9.30%            
Maturity date May 25, 2029   May 25, 2029 May 25, 2029 May 25, 2029 May 25, 2029            
Principal/Shares $ 24,696                      
Cost 24,460                      
Market Value $ 24,449                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Applitools First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Applitools [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Maturity date May 25, 2028   May 25, 2028 May 25, 2028 May 25, 2028 May 25, 2028            
Cost $ (32)                      
Market Value $ (34)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Appriss Holdings, Inc. Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Appriss Holdings Inc [Member] Appriss Holdings Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] Common Stock [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Principal/Shares $ 2,136 $ 2,136                    
Cost 1,606 1,606                    
Market Value $ 1,505 $ 1,552                    
Acquisition date May 03, 2021 May 03, 2021                    
Investment, Restriction Status [true false] true false true true true true false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true false true true true true false false false false false false
Investment, Identifier [Axis]: Appriss Holdings, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Appriss Holdings Inc [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   High Tech Industries [Member]         High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent)   7.25%         7.25% 7.25% 7.25% 7.25% 7.25% 7.25%
Interest rate   8.25%         8.25% 8.25% 8.25% 8.25% 8.25% 8.25%
Maturity date   May 06, 2027         May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027
Principal/Shares   $ 11,292                    
Cost   11,081                    
Market Value   $ 11,179                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Appriss Holdings, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Appriss Holdings Inc [Member] Appriss Holdings Inc [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Maturity date May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027
Cost $ (12) $ (13)                    
Market Value $ (13) $ (8)                    
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Appriss Holdings, Inc. First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Appriss Holdings Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Spread Above Index (as a percent) 7.25%   7.25% 7.25% 7.25% 7.25%            
Interest rate 9.93%   9.93% 9.93% 9.93% 9.93%            
Maturity date May 06, 2027   May 06, 2027 May 06, 2027 May 06, 2027 May 06, 2027            
Principal/Shares $ 11,278                      
Cost 11,092                      
Market Value $ 11,081                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Aptus 1724 Gmbh First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:Aptus1724GmbhMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Media: Diversified & Production   Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production            
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 9.39%   9.39% 9.39% 9.39% 9.39%            
Maturity date Aug. 23, 2027   Aug. 23, 2027 Aug. 23, 2027 Aug. 23, 2027 Aug. 23, 2027            
Principal/Shares $ 4,971                      
Cost 4,971                      
Market Value $ 4,909                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.25%   0.25% 0.25% 0.25% 0.25%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Aptus 1724 Gmbh First Lien Senior Secured Loan Euibor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:Aptus1724GmbhMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Media: Diversified & Production         Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production
Spread Above Index (as a percent)   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate   6.25%         6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date   Feb. 23, 2028         Feb. 23, 2028 Feb. 23, 2028 Feb. 23, 2028 Feb. 23, 2028 Feb. 23, 2028 Feb. 23, 2028
Principal/Shares | €               € 4,162        
Cost | €               5,055        
Market Value | €               € 4,732        
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:EuroInterbankOfferRateMember         bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Aptus 1724 Gmbh First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:Aptus1724GmbhMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Media: Diversified & Production         Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production
Spread Above Index (as a percent)   6.25%         6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Interest rate   6.50%         6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date   Feb. 23, 2028         Feb. 23, 2028 Feb. 23, 2028 Feb. 23, 2028 Feb. 23, 2028 Feb. 23, 2028 Feb. 23, 2028
Principal/Shares   $ 14,971                    
Cost   14,971                    
Market Value   $ 14,971                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Aramsco, Inc First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Aramsco Inc [Member] Aramsco Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member]
Spread Above Index (as a percent) 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25%
Interest rate 8.37% 5.35% 8.37% 8.37% 8.37% 8.37% 5.35% 5.35% 5.35% 5.35% 5.35% 5.35%
Maturity date Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024
Principal/Shares $ 14,103 $ 23,796                    
Cost 13,979 23,537                    
Market Value $ 14,103 $ 23,796                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Aramsco, Inc First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Aramsco Inc [Member] Aramsco Inc [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member]
Spread Above Index (as a percent) 5.25%   5.25% 5.25% 5.25% 5.25%            
Interest rate 8.37%   8.37% 8.37% 8.37% 8.37%            
Maturity date Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024 Aug. 28, 2024
Principal/Shares $ 226                      
Cost 201                      
Cost   $ (30)                    
Market Value $ 226                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Armor Group, LP Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ArmorGroupLPMember bcsf:ArmorGroupLPMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Equity Securities [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member]
Investment, Industry Sector [Extensible Enumeration] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member]
Principal/Shares $ 10 $ 10                    
Cost 1,012 1,012                    
Market Value $ 2,086 $ 2,131                    
Acquisition date Aug. 28, 2018 Aug. 28, 2018                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Armstrong Bidco Limited First Lien Senior Secured Loan Sonia                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:ArmstrongBidcoLimitedMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   High Tech Industries [Member]         High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent)   4.75%         4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
Interest rate   5.00%         5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Maturity date   Apr. 30, 2025         Apr. 30, 2025 Apr. 30, 2025 Apr. 30, 2025 Apr. 30, 2025 Apr. 30, 2025 Apr. 30, 2025
Principal/Shares   $ 56                    
Cost   78                    
Market Value   $ 76                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]         Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]
Investment, Interest Rate, Floor   0.25%         0.25% 0.25% 0.25% 0.25% 0.25% 0.25%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Armstrong Bidco T/L First Lien Senior Secured Loan Sonia                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Armstrong Bidco T/L                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   High Tech Industries [Member]         High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent)   4.75%         4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
Interest rate   5.06%         5.06% 5.06% 5.06% 5.06% 5.06% 5.06%
Maturity date   Apr. 30, 2025         Apr. 30, 2025 Apr. 30, 2025 Apr. 30, 2025 Apr. 30, 2025 Apr. 30, 2025 Apr. 30, 2025
Principal/Shares | €               € 705        
Cost | €               763        
Market Value | €               € 954        
Investment, Variable Interest Rate, Type [Extensible Enumeration]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]         Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Avalon Acquiror, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Avalon Acquiror, Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 9.80%   9.80% 9.80% 9.80% 9.80%            
Maturity date Mar. 10, 2028   Mar. 10, 2028 Mar. 10, 2028 Mar. 10, 2028 Mar. 10, 2028            
Principal/Shares $ 24,660                      
Cost 24,431                      
Market Value $ 24,229                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Avalon Acquiror, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Avalon Acquiror, Inc [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 9.38%   9.38% 9.38% 9.38% 9.38%            
Maturity date Mar. 10, 2028   Mar. 10, 2028 Mar. 10, 2028 Mar. 10, 2028 Mar. 10, 2028            
Principal/Shares $ 1,050                      
Cost 878                      
Market Value $ 903                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: BCC Jetstream Holdings Aviation (Off I), LLC Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:BCCJetstreamHoldingsAviationOnIILLCMember bcsf:BCCJetstreamHoldingsAviationOffILLCMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Principal/Shares $ 11,863 $ 11,863                    
Cost 11,863 11,863                    
Market Value $ 10,209 $ 10,563                    
Acquisition date Jun. 01, 2017 Jun. 01, 2017                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: BCC Jetstream Holdings Aviation (On II), LLC Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:BCCJetstreamHoldingsAviationOnIILLCMember bcsf:BCCJetstreamHoldingsAviationOnIILLCMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Principal/Shares $ 1,116 $ 1,116                    
Cost $ 1,116 $ 1,116                    
Acquisition date Jun. 01, 2017 Jun. 01, 2017                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: BCC Jetstream Holdings Aviation (On II), LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:BCCJetstreamHoldingsAviationOnIILLCMember bcsf:BCCJetstreamHoldingsAviationOnIILLCMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00%
Interest rate 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00%
Maturity date Jun. 02, 2023 Jun. 02, 2022 Jun. 02, 2023 Jun. 02, 2023 Jun. 02, 2023 Jun. 02, 2023 Jun. 02, 2022 Jun. 02, 2022 Jun. 02, 2022 Jun. 02, 2022 Jun. 02, 2022 Jun. 02, 2022
Principal/Shares $ 8,013 $ 7,377                    
Cost 8,013 7,377                    
Market Value $ 6,399 $ 6,627                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: BCC Middle Market CLO 2018-1, LLC Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member]   Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:BCCMiddleMarketCLO2018-1LLCMember                      
Investment, Type [Extensible Enumeration] Common Stock [Member]   Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Finance Non Controlled Affiliate   Fire Finance Non Controlled Affiliate Fire Finance Non Controlled Affiliate Fire Finance Non Controlled Affiliate Fire Finance Non Controlled Affiliate            
Maturity date Oct. 20, 2030   Oct. 20, 2030 Oct. 20, 2030 Oct. 20, 2030 Oct. 20, 2030            
Principal/Shares $ 25,635                      
Cost 24,051                      
Market Value $ 22,298                      
Acquisition date Feb. 28, 2022                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: Bain Capital Senior Loan Program, LLC Equity Interest Investment Vehicles                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member]   Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]            
Investment, Issuer Name [Extensible Enumeration] Bain Capital Senior Loan Program L L C [Member]                      
Investment, Type [Extensible Enumeration] Equity Interest Investment Vehicles [Member]   Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member]            
Investment, Industry Sector [Extensible Enumeration] Investment Vehicles [Member]   Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member]            
Principal/Shares $ 5,594                      
Cost 5,594                      
Market Value $ 4,063                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] false   false false false false            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Bain Capital Senior Loan Program, LLC Preferred Equity Interest Investment Vehicles                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member]   Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]            
Investment, Issuer Name [Extensible Enumeration] Bain Capital Senior Loan Program L L C [Member]                      
Investment, Type [Extensible Enumeration] bcsf:PreferredEquityInterestInvestmentVehiclesMember   bcsf:PreferredEquityInterestInvestmentVehiclesMember bcsf:PreferredEquityInterestInvestmentVehiclesMember bcsf:PreferredEquityInterestInvestmentVehiclesMember bcsf:PreferredEquityInterestInvestmentVehiclesMember            
Investment, Industry Sector [Extensible Enumeration] Investment Vehicles [Member]   Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member]            
Principal/Shares $ 10                      
Cost 10                      
Market Value $ (433)                      
Acquisition date Dec. 27, 2021                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] false   false false false false            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Bain Capital Senior Loan Program, LLC Subordinated Note Investment Vehicles                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member]   Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]            
Investment, Issuer Name [Extensible Enumeration] Bain Capital Senior Loan Program L L C [Member]                      
Investment, Type [Extensible Enumeration] bcsf:SubordinatedNoteInvestmentVehiclesMember   bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember            
Investment, Industry Sector [Extensible Enumeration] Investment Vehicles [Member]   Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member]            
Spread Above Index (as a percent) 10.00%   10.00% 10.00% 10.00% 10.00%            
Interest rate 10.00%   10.00% 10.00% 10.00% 10.00%            
Maturity date Dec. 27, 2033   Dec. 27, 2033 Dec. 27, 2033 Dec. 27, 2033 Dec. 27, 2033            
Principal/Shares $ 50,995                      
Cost 50,995                      
Market Value $ 50,995                      
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Batteries Plus Holding Corporation First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Batteries Plus Holding Corporation [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Retail Sector [Member]   Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]            
Spread Above Index (as a percent) 6.75%   6.75% 6.75% 6.75% 6.75%            
Interest rate 9.87%   9.87% 9.87% 9.87% 9.87%            
Maturity date Jun. 30, 2023   Jun. 30, 2023 Jun. 30, 2023 Jun. 30, 2023 Jun. 30, 2023            
Principal/Shares $ 18,172                      
Cost 18,172                      
Market Value $ 18,172                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Batteries Plus Holding Corporation First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Batteries Plus Holding Corporation [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Retail Sector [Member]   Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]            
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 12.00%   12.00% 12.00% 12.00% 12.00%            
Maturity date Jun. 30, 2023   Jun. 30, 2023 Jun. 30, 2023 Jun. 30, 2023 Jun. 30, 2023            
Principal/Shares $ 2,038                      
Cost 2,037                      
Market Value $ 2,038                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] us-gaap:PrimeRateMember   us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Batteries Plus Holding Corporation First Lien Senior Secured Loan - Revolver Prime                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Batteries Plus Holding Corporation [Member]                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Retail Sector [Member]         Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   8.44%         8.44% 8.44% 8.44% 8.44% 8.44% 8.44%
Maturity date   Jun. 30, 2023         Jun. 30, 2023 Jun. 30, 2023 Jun. 30, 2023 Jun. 30, 2023 Jun. 30, 2023 Jun. 30, 2023
Principal/Shares   $ 817                    
Cost   817                    
Market Value   $ 817                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   us-gaap:PrimeRateMember         us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Batteries Plus Holding Corporation First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Batteries Plus Holding Corporation [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Retail Sector [Member]         Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]
Spread Above Index (as a percent)   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Interest rate   7.75%         7.75% 7.75% 7.75% 7.75% 7.75% 7.75%
Maturity date   Jun. 30, 2023         Jun. 30, 2023 Jun. 30, 2023 Jun. 30, 2023 Jun. 30, 2023 Jun. 30, 2023 Jun. 30, 2023
Principal/Shares   $ 28,672                    
Cost   28,671                    
Market Value   $ 28,671                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Blackbrush Oil & Gas, L.P. Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:BlackbrushOilGasLPMember bcsf:BlackbrushOilGasLPMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Equity Securities [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member]
Investment, Industry Sector [Extensible Enumeration] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member]
Principal/Shares $ 1,198 $ 1,123                    
Cost $ 1                      
Acquisition date Sep. 03, 2020 Sep. 03, 2020                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Blackbrush Oil & Gas, L.P. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:BlackbrushOilGasLPMember bcsf:BlackbrushOilGasLPMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member]
Spread Above Index (as a percent) 5.00% 5.00% 5.00% 5.00% 5.00% 5.00% 5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
PIK (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%) (2.00%)
Interest rate 9.11% 8.00% 9.11% 9.11% 9.11% 9.11% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00%
Maturity date Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025
Principal/Shares $ 8,995 $ 12,336                    
Cost 8,993 12,336                    
Market Value $ 8,995 $ 12,336                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Blackbrush Oil & Gas, L.P. Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:BlackbrushOilGasLPMember bcsf:BlackbrushOilGasLPMember                    
Investment, Type [Extensible Enumeration] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member] Energy Oil And Gas Noncontrolled Affiliate Investments [Member]
Principal/Shares $ 38,505 $ 36,084                    
Cost 11,777 10,104                    
Market Value $ 29,476 $ 19,720                    
Acquisition date   Sep. 03, 2020                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Brook Bidco Facility B First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:BrookBidcoFacilityBMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate   6.09%         6.09% 6.09% 6.09% 6.09% 6.09% 6.09%
Maturity date   Jul. 07, 2028         Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028
Principal/Shares | £                 £ 684      
Cost | £                 935      
Market Value | £                 £ 926      
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Brook Bidco First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:BrookBidcoMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 3.00%   3.00% 3.00% 3.00% 3.00%            
PIK (4.25%)   (4.25%) (4.25%) (4.25%) (4.25%)            
Interest rate 8.06%   8.06% 8.06% 8.06% 8.06%            
Maturity date Jul. 07, 2028   Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028            
Principal/Shares | £         £ 707              
Cost $ 964                      
Market Value $ 789                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember   bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Brook Bidco I Limited First Lien Senior Secured Loan - Revolver Gbp libor 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:BrookBidcoILimitedMember                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date   Jul. 07, 2028         Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028
Principal/Shares | £                 £ 5,385      
Cost | £                 7,047      
Market Value | £                 £ 7,287      
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember         bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Brook Bidco I Limited First Lien Senior Secured Loan - Revolver Gbp libor 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:BrookBidcoILimitedMember                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date   Jul. 07, 2028         Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028
Principal/Shares | £                 £ 7,180      
Cost | £                 9,396      
Market Value | £                 £ 9,716      
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember         bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Brook Bidco I Limited First Lien Senior Secured Loan, One                        
Consolidated Schedule of Investments                        
PIK 4.25%   4.25% 4.25% 4.25% 4.25%            
Investment, Identifier [Axis]: Brook Bidco I Limited First Lien Senior Secured Loan, Three                        
Consolidated Schedule of Investments                        
PIK 4.25%   4.25% 4.25% 4.25% 4.25%            
Investment, Identifier [Axis]: Brook Bidco I Limited First Lien Senior Secured Loan, Two                        
Consolidated Schedule of Investments                        
PIK 4.25%   4.25% 4.25% 4.25% 4.25%            
Investment, Identifier [Axis]: Brook Bidco Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:BrookBidcoMember                      
Investment, Type [Extensible Enumeration] Preferred Stock [Member]   Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Principal/Shares | £         £ 5,675              
Cost $ 7,783                      
Market Value $ 7,028                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: Brook Bidco Series A Preferred Units Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:BrookBidcoSeriesAPreferredUnitsMember                    
Investment, Type [Extensible Enumeration]   Preferred Stock [Member]         Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Principal/Shares   $ 5,675                    
Cost   7,783                    
Market Value   $ 7,908                    
Acquisition date Jul. 08, 2021 Jul. 08, 2021                    
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   true         true true true true true true
Investment, Identifier [Axis]: CB Nike IntermediateCo Ltd First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] C B Nike Intermediate Co Ltd [Member] C B Nike Intermediate Co Ltd [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
Interest rate 7.56% 5.75% 7.56% 7.56% 7.56% 7.56% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Maturity date Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025
Principal/Shares $ 344 $ 347                    
Cost 341 342                    
Market Value $ 344 $ 347                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: CB Nike IntermediateCo Ltd First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] C B Nike Intermediate Co Ltd [Member] C B Nike Intermediate Co Ltd [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent)   4.75%         4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
Interest rate   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Maturity date Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025
Principal/Shares   $ 44                    
Cost   44                    
Market Value   $ 44                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: CB Titan Holdings, Inc. Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:CBTitanHoldingsIncMember bcsf:CBTitanHoldingsIncMember                    
Investment, Type [Extensible Enumeration] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Principal/Shares $ 1,953 $ 1,953                    
Cost 1,953 1,953                    
Market Value $ 918 $ 1,153                    
Acquisition date May 01, 2017 May 01, 2017                    
Investment, Restriction Status [true false] false true false false false false true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: CPS Group Holdings, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] C P S Group Holdings, Inc [Member] C P S Group Holdings, Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Spread Above Index (as a percent) 5.75% 5.25% 5.75% 5.75% 5.75% 5.75% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25%
Interest rate 9.42% 6.25% 9.42% 9.42% 9.42% 9.42% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025
Principal/Shares $ 44,790 $ 54,843                    
Cost 44,582 54,517                    
Market Value $ 44,790 $ 54,843                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: CPS Group Holdings, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] C P S Group Holdings, Inc [Member] C P S Group Holdings, Inc [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Maturity date Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025 Mar. 03, 2025
Cost $ (43) $ (52)                    
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: CST Buyer Company First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] C S T Buyer Company [Member] C S T Buyer Company [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member]
Spread Above Index (as a percent) 5.50% 5.55% 5.50% 5.50% 5.50% 5.50% 5.55% 5.55% 5.55% 5.55% 5.55% 5.55%
Interest rate 8.62% 6.50% 8.62% 8.62% 8.62% 8.62% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025
Principal/Shares $ 8,322 $ 19,238                    
Cost 8,294 19,122                    
Market Value $ 8,322 $ 19,238                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: CST Buyer Company First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:JHCCHoldingsLLCMember C S T Buyer Company [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member]
Maturity date Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025 Oct. 03, 2025
Cost $ (4) $ (11)                    
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Captain Ds LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Captain D's LLC                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Hotel, Gaming And Leisure [Member]         Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member]
Spread Above Index (as a percent)   4.50%         4.50% 4.50% 4.50% 4.50% 4.50% 4.50%
Interest rate   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Maturity date   Dec. 15, 2023         Dec. 15, 2023 Dec. 15, 2023 Dec. 15, 2023 Dec. 15, 2023 Dec. 15, 2023 Dec. 15, 2023
Principal/Shares   $ 2,326                    
Cost   2,301                    
Market Value   $ 2,326                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Captain D’s LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Captain D's LLC                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Hotel, Gaming And Leisure [Member]         Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member]
Maturity date   Dec. 15, 2023         Dec. 15, 2023 Dec. 15, 2023 Dec. 15, 2023 Dec. 15, 2023 Dec. 15, 2023 Dec. 15, 2023
Cost   $ (6)                    
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Captain D’s LLC First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Captain D's LLC                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Hotel, Gaming And Leisure [Member]         Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member]
Spread Above Index (as a percent)   4.50%         4.50% 4.50% 4.50% 4.50% 4.50% 4.50%
Interest rate   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Maturity date   Dec. 15, 2023         Dec. 15, 2023 Dec. 15, 2023 Dec. 15, 2023 Dec. 15, 2023 Dec. 15, 2023 Dec. 15, 2023
Principal/Shares   $ 12,559                    
Cost   12,539                    
Market Value   $ 12,559                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Cardo First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:CardoMember bcsf:CardoMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member]
Spread Above Index (as a percent) 5.00% 6.00% 5.00% 5.00% 5.00% 5.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate 7.09% 6.50% 7.09% 7.09% 7.09% 7.09% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date May 12, 2028 May 12, 2028 May 12, 2028 May 12, 2028 May 12, 2028 May 12, 2028 May 12, 2028 May 12, 2028 May 12, 2028 May 12, 2028 May 12, 2028 May 12, 2028
Principal/Shares $ 98 $ 10,898                    
Cost 97 10,795                    
Market Value $ 98 $ 10,898                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.50%   0.50% 0.50% 0.50% 0.50%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Caribou Bidco Limited First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Caribou Bidco Limited [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 6.00%   6.00% 6.00% 6.00% 6.00%            
Interest rate 7.19%   7.19% 7.19% 7.19% 7.19%            
Maturity date Jan. 29, 2029   Jan. 29, 2029 Jan. 29, 2029 Jan. 29, 2029 Jan. 29, 2029            
Principal/Shares | £         £ 8,070              
Cost $ 10,799                      
Market Value $ 9,014                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember   bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Caribou Bidco Limited First Lien Senior Secured Loan Not Qualifying                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Caribou Bidco Limited [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 6.00%   6.00% 6.00% 6.00% 6.00%            
Interest rate 7.19%   7.19% 7.19% 7.19% 7.19%            
Maturity date Jan. 29, 2029   Jan. 29, 2029 Jan. 29, 2029 Jan. 29, 2029 Jan. 29, 2029            
Principal/Shares | £         £ 16              
Cost $ 20                      
Market Value $ 18                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember   bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Chamber Bidco Limited First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ChamberBidcoLimitedMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 7.85%   7.85% 7.85% 7.85% 7.85%            
Maturity date Jun. 07, 2028   Jun. 07, 2028 Jun. 07, 2028 Jun. 07, 2028 Jun. 07, 2028            
Principal/Shares $ 237                      
Cost 235                      
Market Value $ 237                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.50%   0.50% 0.50% 0.50% 0.50%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Chamber Bidco Limited First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:ChamberBidcoLimitedMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate   6.50%         6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date   Jun. 07, 2028         Jun. 07, 2028 Jun. 07, 2028 Jun. 07, 2028 Jun. 07, 2028 Jun. 07, 2028 Jun. 07, 2028
Principal/Shares   $ 237                    
Cost   234                    
Market Value   $ 237                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Chase Industries, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ChaseIndustriesIncMember bcsf:ChaseIndustriesIncMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Spread Above Index (as a percent) 7.00% 5.50% 7.00% 7.00% 7.00% 7.00% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
PIK 7.00% (1.50%) 7.00% 7.00% 7.00% 7.00% (1.50%) (1.50%) (1.50%) (1.50%) (1.50%) (1.50%)
Interest rate 9.88% 6.50% 9.88% 9.88% 9.88% 9.88% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025
Principal/Shares $ 13,828 $ 12,622                    
Cost 13,801 12,586                    
Market Value $ 11,063 $ 9,971                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Chase Industries, Inc.First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ChaseIndustriesIncMember bcsf:ChaseIndustriesIncMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Spread Above Index (as a percent) 7.00% 5.50% 7.00% 7.00% 7.00% 7.00% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
PIK 7.00% (1.50%) 7.00% 7.00% 7.00% 7.00% (1.50%) (1.50%) (1.50%) (1.50%) (1.50%) (1.50%)
Interest rate 9.88% 6.50% 9.88% 9.88% 9.88% 9.88% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025 May 12, 2025
Principal/Shares $ 1,307 $ 1,197                    
Cost 1,306 1,195                    
Market Value $ 1,046 $ 946                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: ClockSpring Second Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ClockSpringMember                      
Investment, Type [Extensible Enumeration] Second Lien Senior Secured Loan [Member]   Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Capital Equipment Sector [Member]   Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member]            
Spread Above Index (as a percent) 6.50%   6.50% 6.50% 6.50% 6.50%            
Interest rate 10.05%   10.05% 10.05% 10.05% 10.05%            
Maturity date Aug. 01, 2025   Aug. 01, 2025 Aug. 01, 2025 Aug. 01, 2025 Aug. 01, 2025            
Principal/Shares $ 5,235                      
Cost 5,145                      
Market Value $ 5,131                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Concert Golf Partners Holdco First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ConcertGolfPartnersHoldcoMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Hotel, Gaming And Leisure [Member]   Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 6.50%   6.50% 6.50% 6.50% 6.50%            
Maturity date Mar. 30, 2029   Mar. 30, 2029 Mar. 30, 2029 Mar. 30, 2029 Mar. 30, 2029            
Principal/Shares $ 6,847                      
Cost 6,724                      
Market Value $ 6,778                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Concert Golf Partners Holdco LLC First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ConcertGolfPartnersHoldcoLlcMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Hotel, Gaming And Leisure [Member]   Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 9.40%   9.40% 9.40% 9.40% 9.40%            
Maturity date Mar. 30, 2029   Mar. 30, 2029 Mar. 30, 2029 Mar. 30, 2029 Mar. 30, 2029            
Principal/Shares $ 1,065                      
Cost 987                      
Market Value $ 1,023                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Concert Golf Partners Holdco LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ConcertGolfPartnersHoldcoLlcMember                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Hotel, Gaming And Leisure [Member]   Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 9.46%   9.46% 9.46% 9.46% 9.46%            
Maturity date Mar. 31, 2028   Mar. 31, 2028 Mar. 31, 2028 Mar. 31, 2028 Mar. 31, 2028            
Principal/Shares $ 356                      
Cost 310                      
Market Value $ 331                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Conterra Ultra Broadband Holdings, Inc First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:ConterraUltraBroadbandHoldingsIncMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Telecommunications [Member]         Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]
Spread Above Index (as a percent)   4.75%         4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
Interest rate   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Maturity date   Apr. 30, 2026         Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026
Principal/Shares   $ 6,321                    
Cost   6,300                    
Market Value   $ 6,332                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: DC Blox Inc First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:DCBloxIncMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Telecommunications [Member]         Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]
Spread Above Index (as a percent)   8.00%         8.00% 8.00% 8.00% 8.00% 8.00% 8.00%
PIK   (6.00%)         (6.00%) (6.00%) (6.00%) (6.00%) (6.00%) (6.00%)
Interest rate   9.00%         9.00% 9.00% 9.00% 9.00% 9.00% 9.00%
Maturity date   Mar. 22, 2026         Mar. 22, 2026 Mar. 22, 2026 Mar. 22, 2026 Mar. 22, 2026 Mar. 22, 2026 Mar. 22, 2026
Principal/Shares   $ 16,998                    
Cost   16,738                    
Market Value   $ 16,998                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: DC Blox Inc Warrants                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:DCBloxIncMember                    
Investment, Type [Extensible Enumeration]   Warrant [Member]         Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member]
Investment, Industry Sector [Extensible Enumeration]   Telecommunications [Member]         Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]
Principal/Shares   $ 177                    
Cost   $ 2                    
Investment, Restriction Status [true false]   true         true true true true true true
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   true         true true true true true true
Investment, Identifier [Axis]: DC Blox Inc,Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:DCBloxIncMember                      
Investment, Type [Extensible Enumeration] Preferred Stock [Member]   Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Telecommunications [Member]   Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]            
Principal/Shares $ 3,822                      
Cost 3,851                      
Market Value $ 4,449                      
Acquisition date Mar. 22, 2021                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: DC Blox Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:DCBloxIncMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Telecommunications [Member]   Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]            
Spread Above Index (as a percent) 2.00%   2.00% 2.00% 2.00% 2.00%            
PIK (6.00%)   (6.00%) (6.00%) (6.00%) (6.00%)            
Interest rate 11.29%   11.29% 11.29% 11.29% 11.29%            
Maturity date Mar. 22, 2026   Mar. 22, 2026 Mar. 22, 2026 Mar. 22, 2026 Mar. 22, 2026            
Principal/Shares $ 25,404                      
Cost 25,185                      
Market Value $ 25,404                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: DC Blox Inc. Warrants                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:DCBloxIncMember                      
Investment, Type [Extensible Enumeration] Warrant [Member]   Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member]            
Investment, Industry Sector [Extensible Enumeration] Telecommunications [Member]   Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]            
Principal/Shares $ 177                      
Cost $ 2                      
Acquisition date   Mar. 23, 2021                    
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Darcy Partners Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Darcy Partners [Member]                      
Investment, Type [Extensible Enumeration] Common Stock [Member]   Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Principal/Shares $ 359                      
Cost 359                      
Market Value $ 367                      
Acquisition date Jun. 01, 2022                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Identifier [Axis]: Darcy Partners First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Darcy Partners [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 7.75%   7.75% 7.75% 7.75% 7.75%            
Interest rate 9.50%   9.50% 9.50% 9.50% 9.50%            
Maturity date Jun. 01, 2028   Jun. 01, 2028 Jun. 01, 2028 Jun. 01, 2028 Jun. 01, 2028            
Principal/Shares $ 1,530                      
Cost 1,515                      
Market Value $ 1,515                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.50%   1.50% 1.50% 1.50% 1.50%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Darcy Partners First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Darcy Partners [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Maturity date Jun. 01, 2028   Jun. 01, 2028 Jun. 01, 2028 Jun. 01, 2028 Jun. 01, 2028            
Market Value $ (3)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Datix Bidco Limited First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:DatixBidcoLimitedMember bcsf:DatixBidcoLimitedMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Spread Above Index (as a percent) 4.50% 4.00% 4.50% 4.50% 4.50% 4.50% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00%
Interest rate 4.57% 4.25% 4.57% 4.57% 4.57% 4.57% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25%
Maturity date Apr. 28, 2025 Apr. 28, 2025 Apr. 28, 2025 Apr. 28, 2025 Apr. 28, 2025 Apr. 28, 2025 Apr. 28, 2025 Apr. 28, 2025 Apr. 28, 2025 Apr. 28, 2025 Apr. 28, 2025 Apr. 28, 2025
Principal/Shares     $ 42       $ 42          
Cost $ 32           32          
Market Value $ 26           $ 31          
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:BankBillSwapRateMember bcsf:BankBillSwapRateMember bcsf:BankBillSwapRateMember bcsf:BankBillSwapRateMember bcsf:BankBillSwapRateMember bcsf:BankBillSwapRateMember bcsf:BankBillSwapRateMember bcsf:BankBillSwapRateMember bcsf:BankBillSwapRateMember bcsf:BankBillSwapRateMember bcsf:BankBillSwapRateMember bcsf:BankBillSwapRateMember
Investment, Interest Rate, Floor 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Datix Bidco Limited First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:DatixBidcoLimitedMember                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Healthcare And Pharmaceuticals [Member]         Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Spread Above Index (as a percent)   4.50%         4.50% 4.50% 4.50% 4.50% 4.50% 4.50%
Interest rate   4.96%         4.96% 4.96% 4.96% 4.96% 4.96% 4.96%
Maturity date   Oct. 28, 2024         Oct. 28, 2024 Oct. 28, 2024 Oct. 28, 2024 Oct. 28, 2024 Oct. 28, 2024 Oct. 28, 2024
Principal/Shares | €               € 10        
Cost | €               13        
Market Value | €               € 13        
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   0.00%         0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Datix Bidco Limited Second Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:DatixBidcoLimitedMember bcsf:DatixBidcoLimitedMember                    
Investment, Type [Extensible Enumeration] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Spread Above Index (as a percent) 7.75% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75%
Interest rate 9.44% 8.21% 9.44% 9.44% 9.44% 9.44% 8.21% 8.21% 8.21% 8.21% 8.21% 8.21%
Maturity date Apr. 27, 2026 Apr. 27, 2026 Apr. 27, 2026 Apr. 27, 2026 Apr. 27, 2026 Apr. 27, 2026 Apr. 27, 2026 Apr. 27, 2026 Apr. 27, 2026 Apr. 27, 2026 Apr. 27, 2026 Apr. 27, 2026
Principal/Shares         £ 121     € 121        
Cost $ 164             164        
Market Value $ 132             € 164        
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember London Interbank Offered Rate (LIBOR) [Member] bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Direct Travel, Inc Equity Interest                        
Consolidated Schedule of Investments                        
Acquisition date   Oct. 02, 2020                    
Investment, Identifier [Axis]: Direct Travel, Inc First Lien Senior Secured Loan - Delayed Draw 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member]   Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]            
Investment, Issuer Name [Extensible Enumeration] Direct Travel, Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Transportation Consumer Non Controlled Affiliate [Member]   Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member]            
Spread Above Index (as a percent) 8.50%   8.50% 8.50% 8.50% 8.50%            
Interest rate 12.17%   12.17% 12.17% 12.17% 12.17%            
Maturity date Oct. 02, 2023   Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023            
Principal/Shares $ 3,440                      
Cost 3,440                      
Market Value $ 3,371                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Direct Travel, Inc First Lien Senior Secured Loan - Delayed Draw 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member]   Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]            
Investment, Issuer Name [Extensible Enumeration] Direct Travel, Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Transportation Consumer Non Controlled Affiliate [Member]   Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member]            
Spread Above Index (as a percent) 8.50%   8.50% 8.50% 8.50% 8.50%            
Interest rate 12.17%   12.17% 12.17% 12.17% 12.17%            
Maturity date Oct. 02, 2023   Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023            
Principal/Shares $ 1,741                      
Cost 1,741                      
Market Value $ 1,707                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Direct Travel, Inc First Lien Senior Secured Loan - Delayed Draw 3                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member]   Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]            
Investment, Issuer Name [Extensible Enumeration] Direct Travel, Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Transportation Consumer Non Controlled Affiliate [Member]   Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member]            
Spread Above Index (as a percent) 6.00%   6.00% 6.00% 6.00% 6.00%            
Interest rate 8.28%   8.28% 8.28% 8.28% 8.28%            
Maturity date Oct. 02, 2023   Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023            
Principal/Shares $ 4,125                      
Cost 4,125                      
Market Value $ 4,125                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Direct Travel, Inc First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Investment, Issuer Name [Extensible Enumeration] Direct Travel, Inc [Member] Direct Travel, Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member]
Spread Above Index (as a percent) 8.50% 1.00% 8.50% 8.50% 8.50% 8.50% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
PIK   (8.28%)         (8.28%) (8.28%) (8.28%) (8.28%) (8.28%) (8.28%)
Interest rate 12.17% 9.50% 12.17% 12.17% 12.17% 12.17% 9.50% 9.50% 9.50% 9.50% 9.50% 9.50%
Maturity date Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023
Principal/Shares $ 58,721 $ 57,555                    
Cost 58,721 57,555                    
Market Value $ 57,547 $ 48,347                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Direct Travel, Inc First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Investment, Issuer Name [Extensible Enumeration] Direct Travel, Inc [Member] Direct Travel, Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member]
Spread Above Index (as a percent) 6.50% 6.00% 6.50% 6.50% 6.50% 6.50% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate 10.17% 7.00% 10.17% 10.17% 10.17% 10.17% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023
Principal/Shares $ 4,841 $ 202                    
Cost 4,841 202                    
Market Value $ 4,841 $ 202                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Direct Travel, Inc First Lien Senior Secured Loan 3                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member]   Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]            
Investment, Issuer Name [Extensible Enumeration] Direct Travel, Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Transportation Consumer Non Controlled Affiliate [Member]   Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member]            
Spread Above Index (as a percent) 6.00%   6.00% 6.00% 6.00% 6.00%            
Interest rate 9.67%   9.67% 9.67% 9.67% 9.67%            
Maturity date Oct. 02, 2023   Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023            
Principal/Shares $ 202                      
Cost 202                      
Market Value $ 202                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Direct Travel, Inc. Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Investment, Issuer Name [Extensible Enumeration] Direct Travel, Inc [Member] Direct Travel, Inc [Member]                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Equity Securities [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member]
Principal/Shares $ 68 $ 68                    
Acquisition date Oct. 02, 2020                      
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Direct Travel, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Affiliated Issuer, Noncontrolled [Member]         Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Investment, Issuer Name [Extensible Enumeration]   Direct Travel, Inc [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Transportation Consumer Non Controlled Affiliate [Member]         Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member]
Spread Above Index (as a percent)   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
PIK   (6.30%)         (6.30%) (6.30%) (6.30%) (6.30%) (6.30%) (6.30%)
Interest rate   7.50%         7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Maturity date   Oct. 02, 2023         Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023
Principal/Shares   $ 4,766                    
Cost   4,766                    
Market Value   $ 4,766                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Direct Travel, Inc. First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Affiliated Issuer, Noncontrolled [Member]         Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Investment, Issuer Name [Extensible Enumeration]   Direct Travel, Inc [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Transportation Consumer Non Controlled Affiliate [Member]         Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member]
Spread Above Index (as a percent)   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
PIK   (8.28%)         (8.28%) (8.28%) (8.28%) (8.28%) (8.28%) (8.28%)
Interest rate   9.50%         9.50% 9.50% 9.50% 9.50% 9.50% 9.50%
Maturity date   Oct. 02, 2023         Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023
Principal/Shares   $ 3,370                    
Cost   3,370                    
Market Value   $ 2,831                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Direct Travel, Inc. First Lien Senior Secured Loan - Delayed Draw 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Affiliated Issuer, Noncontrolled [Member]         Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Investment, Issuer Name [Extensible Enumeration]   Direct Travel, Inc [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Transportation Consumer Non Controlled Affiliate [Member]         Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member]
Spread Above Index (as a percent)   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
PIK   (8.28%)         (8.28%) (8.28%) (8.28%) (8.28%) (8.28%) (8.28%)
Interest rate   9.50%         9.50% 9.50% 9.50% 9.50% 9.50% 9.50%
Maturity date   Oct. 02, 2023         Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023
Principal/Shares   $ 1,710                    
Cost   1,710                    
Market Value   $ 1,436                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Direct Travel, Inc. First Lien Senior Secured Loan - Delayed Draw 3                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Affiliated Issuer, Noncontrolled [Member]         Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member] Investment, Affiliated Issuer, Noncontrolled [Member]
Investment, Issuer Name [Extensible Enumeration]   Direct Travel, Inc [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Transportation Consumer Non Controlled Affiliate [Member]         Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member] Transportation Consumer Non Controlled Affiliate [Member]
Spread Above Index (as a percent)   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate   7.00%         7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date   Oct. 02, 2023         Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023 Oct. 02, 2023
Principal/Shares   $ 4,125                    
Cost   4,125                    
Market Value   $ 4,125                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Drilling Info Holdings, Inc First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:DrillingInfoHoldingsIncMember bcsf:DrillingInfoHoldingsIncMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 4.25% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25%
Interest rate 7.37% 4.35% 7.37% 7.37% 7.37% 7.37% 4.35% 4.35% 4.35% 4.35% 4.35% 4.35%
Maturity date Jul. 30, 2025 Jul. 30, 2025 Jul. 30, 2025 Jul. 30, 2025 Jul. 30, 2025 Jul. 30, 2025 Jul. 30, 2025 Jul. 30, 2025 Jul. 30, 2025 Jul. 30, 2025 Jul. 30, 2025 Jul. 30, 2025
Principal/Shares $ 11,178 $ 22,152                    
Cost 11,160 22,101                    
Market Value $ 10,815 $ 21,930                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.00% 1.00% 0.00% 0.00% 0.00% 0.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true false true true true true false false false false false false
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Eagle Rock Capital Corporation Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:EagleRockCapitalCorporationMember bcsf:EagleRockCapitalCorporationMember                    
Investment, Type [Extensible Enumeration] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Principal/Shares $ 3,345 $ 2,354                    
Cost 3,345 2,354                    
Market Value $ 3,798 $ 2,354                    
Acquisition date Dec. 09, 2021 Dec. 09, 2021                    
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] true false true true true true false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true false true true true true false false false false false false
Investment, Identifier [Axis]: East BCC Coinvest II, LLC Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:EastBCCCoinvestIILLCMember bcsf:EastBCCCoinvestIILLCMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member]
Principal/Shares $ 1,419 $ 1,419                    
Cost 1,419 1,419                    
Market Value $ 989 $ 1,065                    
Acquisition date Jul. 23, 2019 Jul. 23, 2019                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Efficient Collaborative Retail Marketing Company, LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Efficient Collaborative Retail Marketing Company, L L C [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Media: Diversified & Production   Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production            
Spread Above Index (as a percent) 5.25%   5.25% 5.25% 5.25% 5.25%            
Interest rate 8.73%   8.73% 8.73% 8.73% 8.73%            
Maturity date Jun. 15, 2024   Jun. 15, 2024 Jun. 15, 2024 Jun. 15, 2024 Jun. 15, 2024            
Principal/Shares $ 1,275                      
Cost 1,275                      
Market Value $ 1,275                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Efficient Collaborative Retail Marketing Company, LLC First Lien Senior Secured Loan - Revolver Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Efficient Collaborative Retail Marketing Company, L L C [Member]                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Media: Diversified & Production         Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production
Spread Above Index (as a percent)   5.25%         5.25% 5.25% 5.25% 5.25% 5.25% 5.25%
Interest rate   6.25%         6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date   Jun. 15, 2022         Jun. 15, 2022 Jun. 15, 2022 Jun. 15, 2022 Jun. 15, 2022 Jun. 15, 2022 Jun. 15, 2022
Principal/Shares   $ 1,275                    
Cost   1,275                    
Market Value   $ 1,275                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Efficient Collaborative Retail Marketing Company, LLC First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Efficient Collaborative Retail Marketing Company, L L C [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Media: Diversified & Production   Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production            
Spread Above Index (as a percent) 6.75%   6.75% 6.75% 6.75% 6.75%            
Interest rate 10.42%   10.42% 10.42% 10.42% 10.42%            
Maturity date Jun. 15, 2024   Jun. 15, 2024 Jun. 15, 2024 Jun. 15, 2024 Jun. 15, 2024            
Principal/Shares $ 15,006                      
Cost 15,006                      
Market Value $ 13,543                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Efficient Collaborative Retail Marketing Company, LLC First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Efficient Collaborative Retail Marketing Company, L L C [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Media: Diversified & Production   Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production            
Spread Above Index (as a percent) 6.75%   6.75% 6.75% 6.75% 6.75%            
Interest rate 10.42%   10.42% 10.42% 10.42% 10.42%            
Maturity date Jun. 15, 2024   Jun. 15, 2024 Jun. 15, 2024 Jun. 15, 2024 Jun. 15, 2024            
Principal/Shares $ 9,736                      
Cost 9,764                      
Market Value $ 8,787                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Efficient Collaborative Retail Marketing Company, LLC First Lien Senior Secured Loan Libor 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Efficient Collaborative Retail Marketing Company, L L C [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Retail Sector [Member]         Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]
Spread Above Index (as a percent)   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Interest rate   7.75%         7.75% 7.75% 7.75% 7.75% 7.75% 7.75%
Maturity date   Jun. 15, 2022         Jun. 15, 2022 Jun. 15, 2022 Jun. 15, 2022 Jun. 15, 2022 Jun. 15, 2022 Jun. 15, 2022
Principal/Shares   $ 15,095                    
Cost   15,114                    
Market Value   $ 14,340                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Efficient Collaborative Retail Marketing Company, LLC First Lien Senior Secured Loan Libor 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Efficient Collaborative Retail Marketing Company, L L C [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Media: Diversified & Production         Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production
Spread Above Index (as a percent)   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Interest rate   7.75%         7.75% 7.75% 7.75% 7.75% 7.75% 7.75%
Maturity date   Jun. 15, 2022         Jun. 15, 2022 Jun. 15, 2022 Jun. 15, 2022 Jun. 15, 2022 Jun. 15, 2022 Jun. 15, 2022
Principal/Shares   $ 9,788                    
Cost   9,800                    
Market Value   $ 9,298                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Electronics For Imaging, Inc. Second Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:ElectronicsForImagingIncMember                    
Investment, Type [Extensible Enumeration]   Second Lien Senior Secured Loan [Member]         Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Capital Equipment Sector [Member]         Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member]
Spread Above Index (as a percent)   9.00%         9.00% 9.00% 9.00% 9.00% 9.00% 9.00%
Interest rate   9.10%         9.10% 9.10% 9.10% 9.10% 9.10% 9.10%
Maturity date   Jul. 23, 2027         Jul. 23, 2027 Jul. 23, 2027 Jul. 23, 2027 Jul. 23, 2027 Jul. 23, 2027 Jul. 23, 2027
Principal/Shares   $ 12,070                    
Cost   11,460                    
Market Value   $ 11,285                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   0.00%         0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Element Buyer, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Element Buyer, Inc [Member] Element Buyer, Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 8.62% 6.50% 8.62% 8.62% 8.62% 8.62% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025
Principal/Shares $ 36,721 $ 37,007                    
Cost 36,878 37,199                    
Market Value $ 36,721 $ 37,007                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Element Buyer, Inc. First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Element Buyer, Inc [Member] Element Buyer, Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 8.62% 6.50% 8.62% 8.62% 8.62% 8.62% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Jul. 19, 2025 Jul. 18, 2025 Jul. 19, 2025 Jul. 19, 2025 Jul. 19, 2025 Jul. 19, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025 Jul. 18, 2025
Principal/Shares $ 10,993 $ 11,078                    
Cost 11,008 11,097                    
Market Value $ 10,993 $ 11,078                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Element Buyer, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Element Buyer, Inc [Member] Element Buyer, Inc [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 8.62% 6.50% 8.62% 8.62% 8.62% 8.62% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Jul. 19, 2024 Jul. 19, 2024 Jul. 19, 2024 Jul. 19, 2024 Jul. 19, 2024 Jul. 19, 2024 Jul. 19, 2024 Jul. 19, 2024 Jul. 19, 2024 Jul. 19, 2024 Jul. 19, 2024 Jul. 19, 2024
Principal/Shares $ 850 $ 1,700                    
Cost 831 1,672                    
Market Value $ 850 $ 1,700                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Elevator Holdco Inc. Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ElevatorHoldcoIncMember bcsf:ElevatorHoldcoIncMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Principal/Shares $ 2 $ 2                    
Cost 2,448 2,448                    
Market Value $ 3,083 $ 2,550                    
Acquisition date Dec. 23, 2019 Dec. 23, 2019                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Eleven Software First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Eleven Software [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Spread Above Index (as a percent) 8.00%   8.00% 8.00% 8.00% 8.00%            
Interest rate 11.55%   11.55% 11.55% 11.55% 11.55%            
Maturity date Apr. 25, 2027   Apr. 25, 2027 Apr. 25, 2027 Apr. 25, 2027 Apr. 25, 2027            
Principal/Shares $ 7,439                      
Cost 7,369                      
Market Value $ 7,365                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Eleven Software First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Eleven Software [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Maturity date Sep. 25, 2026   Sep. 25, 2026 Sep. 25, 2026 Sep. 25, 2026 Sep. 25, 2026            
Cost $ (13)                      
Market Value $ (15)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Eleven Software Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Eleven Software [Member]                      
Investment, Type [Extensible Enumeration] Preferred Stock [Member]   Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Principal/Shares $ 896                      
Cost 896                      
Market Value $ 896                      
Acquisition date Apr. 25, 2022                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: Elk Parent Holdings, LP Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ElkParentHoldingsLPMember bcsf:ElkParentHoldingsLPMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Principal/Shares $ 1 $ 1                    
Cost 12 12                    
Market Value $ 487 $ 407                    
Acquisition date Nov. 01, 2019 Nov. 01, 2019                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Elk Parent Holdings, LP Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ElkParentHoldingsLPMember bcsf:ElkParentHoldingsLPMember                    
Investment, Type [Extensible Enumeration] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Principal/Shares $ 120 $ 120                    
Cost 1,202 1,202                    
Market Value $ 1,514 $ 1,427                    
Acquisition date   Nov. 01, 2019                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Ergotron Acquisition Llc First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ErgotronAcquisitionLlcMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Capital Equipment Sector [Member]   Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 8.89%   8.89% 8.89% 8.89% 8.89%            
Maturity date Jul. 06, 2028   Jul. 06, 2028 Jul. 06, 2028 Jul. 06, 2028 Jul. 06, 2028            
Principal/Shares $ 12,250                      
Cost 12,010                      
Market Value $ 12,005                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: FCG Acquisitions, Inc. Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:FCGAcquisitionsIncMember bcsf:FCGAcquisitionsIncMember                    
Investment, Type [Extensible Enumeration] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member]
Principal/Shares $ 4 $ 4                    
Acquisition date Jan. 24, 2019 Jan. 24, 2019                    
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: FL Hawk Intermediate Holdings, Inc. Second Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:FLHawkIntermediateHoldingsIncMember bcsf:FLHawkIntermediateHoldingsIncMember                    
Investment, Type [Extensible Enumeration] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]
Spread Above Index (as a percent) 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00%
Interest rate 12.42% 10.00% 12.42% 12.42% 12.42% 12.42% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00%
Maturity date Aug. 22, 2028 Aug. 22, 2028 Aug. 22, 2028 Aug. 22, 2028 Aug. 22, 2028 Aug. 22, 2028 Aug. 22, 2028 Aug. 22, 2028 Aug. 22, 2028 Aug. 22, 2028 Aug. 22, 2028 Aug. 22, 2028
Principal/Shares $ 15,125 $ 21,125                    
Cost 14,744 20,543                    
Market Value $ 15,125 $ 21,125                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: FNZ UK Finco Limited First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:FnzUkFincoLimitedMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Finance [Member]   Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member]            
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 7.76%   7.76% 7.76% 7.76% 7.76%            
Maturity date Sep. 30, 2026   Sep. 30, 2026 Sep. 30, 2026 Sep. 30, 2026 Sep. 30, 2026            
Principal/Shares $ 7,741                      
Cost 5,276                      
Market Value $ 4,830                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Fineline Technologies, Inc. Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:FinelineTechnologiesIncMember bcsf:FinelineTechnologiesIncMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]
Principal/Shares $ 939 $ 939                    
Cost 939 939                    
Market Value $ 1,201 $ 1,241                    
Acquisition date Feb. 22, 2021 Feb. 22, 2021                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Forming Machining Industries Holdings, LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:FormingMachiningIndustriesHoldingsLLCMember bcsf:FormingMachiningIndustriesHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 4.25% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25% 4.25%
Interest rate 7.37% 4.47% 7.37% 7.37% 7.37% 7.37% 4.47% 4.47% 4.47% 4.47% 4.47% 4.47%
Maturity date Oct. 09, 2025 Oct. 09, 2025 Oct. 09, 2025 Oct. 09, 2025 Oct. 09, 2025 Oct. 09, 2025 Oct. 09, 2025 Oct. 09, 2025 Oct. 09, 2025 Oct. 09, 2025 Oct. 09, 2025 Oct. 09, 2025
Principal/Shares $ 16,312 $ 16,439                    
Cost 16,242 16,352                    
Market Value $ 14,028 $ 15,288                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true false true true true true false false false false false false
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Forming Machining Industries Holdings, LLC Second Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:FormingMachiningIndustriesHoldingsLLCMember bcsf:FormingMachiningIndustriesHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 8.25% 8.25% 8.25% 8.25% 8.25% 8.25% 8.25% 8.25% 8.25% 8.25% 8.25% 8.25%
Interest rate 11.37% 8.47% 11.37% 11.37% 11.37% 11.37% 8.47% 8.47% 8.47% 8.47% 8.47% 8.47%
Maturity date Oct. 09, 2026 Oct. 09, 2026 Oct. 09, 2026 Oct. 09, 2026 Oct. 09, 2026 Oct. 09, 2026 Oct. 09, 2026 Oct. 09, 2026 Oct. 09, 2026 Oct. 09, 2026 Oct. 09, 2026 Oct. 09, 2026
Principal/Shares $ 6,540 $ 6,540                    
Cost 6,501 6,494                    
Market Value $ 5,297 $ 5,821                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: GSP Holdings LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:GSPHoldingsLLCMember bcsf:GSPHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
PIK (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%)
Interest rate 9.42% 6.75% 9.42% 9.42% 9.42% 9.42% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date Nov. 06, 2025 Nov. 06, 2024 Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2024 Nov. 06, 2024 Nov. 06, 2024 Nov. 06, 2024 Nov. 06, 2024 Nov. 06, 2024
Principal/Shares $ 35,548 $ 35,622                    
Cost 35,523 35,516                    
Market Value $ 33,415 $ 32,951                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: GSP Holdings LLC Senior First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:GSPHoldingsLLCMember bcsf:GSPHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
PIK (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%) (0.25%)
Interest rate 8.77% 6.75% 8.77% 8.77% 8.77% 8.77% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2025 Nov. 06, 2025
Principal/Shares $ 4,096 $ 1,602                    
Cost 4,073 1,573                    
Market Value $ 3,823 $ 1,261                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Gale Aviation (Offshore) Co Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]
Investment, Issuer Name [Extensible Enumeration] Gale Aviation Offshore Co [Member] Gale Aviation Offshore Co [Member]                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Principal/Shares $ 90,450 $ 88,985                    
Cost 90,450 88,985                    
Market Value $ 88,695 $ 72,839                    
Acquisition date Jan. 02, 2019 Jan. 02, 2019                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true false true true true true false false false false false false
Investment, Identifier [Axis]: Gluware First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:GluwareMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Spread Above Index (as a percent) 9.00%   9.00% 9.00% 9.00% 9.00%            
PIK (3.50%)   (3.50%) (3.50%) (3.50%) (3.50%)            
Interest rate 12.50%   12.50% 12.50% 12.50% 12.50%            
Maturity date Oct. 15, 2025   Oct. 15, 2025 Oct. 15, 2025 Oct. 15, 2025 Oct. 15, 2025            
Principal/Shares $ 19,404                      
Cost 18,700                      
Market Value $ 18,434                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Gluware T/L First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:GluwareTLMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   High Tech Industries [Member]         High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent)   12.50%         12.50% 12.50% 12.50% 12.50% 12.50% 12.50%
Interest rate   9.00%         9.00% 9.00% 9.00% 9.00% 9.00% 9.00%
Maturity date   Oct. 15, 2025         Oct. 15, 2025 Oct. 15, 2025 Oct. 15, 2025 Oct. 15, 2025 Oct. 15, 2025 Oct. 15, 2025
Principal/Shares   $ 18,898                    
Cost   18,534                    
Market Value   $ 18,520                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Gluware Warrant Warrants                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:GluwareWarrantMember                    
Investment, Type [Extensible Enumeration]   Warrant [Member]         Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member]
Investment, Industry Sector [Extensible Enumeration]   High Tech Industries [Member]         High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Principal/Shares   $ 3,328                    
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Gluware Warrants                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:GluwareMember                      
Investment, Type [Extensible Enumeration] Warrant [Member]   Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Principal/Shares $ 3,328                      
Cost 478                      
Market Value $ 435                      
Acquisition date Oct. 15, 2021 Oct. 15, 2021                    
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: Goldman Sachs Financial Square Government Fund Institutional Share Class Cash Equivalents                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member] Cash Equivalents [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:GoldmanSachsFinancialSquareGovernmentFundInstitutionalShareClassMember bcsf:GoldmanSachsFinancialSquareGovernmentFundInstitutionalShareClassMember                    
Investment, Type [Extensible Enumeration] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member]
Investment, Industry Sector [Extensible Enumeration] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member] Cash Equivalents [Member]
Interest rate 0.03% 0.03% 0.03% 0.03% 0.03% 0.03% 0.03% 0.03% 0.03% 0.03% 0.03% 0.03%
Principal/Shares $ 31,249 $ 177,554                    
Cost 31,249 177,554                    
Market Value $ 31,249 $ 177,554                    
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] false false false false false false false false false false false false
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Grammer Investment Holdings LLC Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:GrammerInvestmentHoldingsLLCMember bcsf:GrammerInvestmentHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
PIK 10.00%   10.00% 10.00% 10.00% 10.00%            
Principal/Shares $ 1,011 $ 1,011                    
Cost 1,011 1,011                    
Market Value $ 1,011 $ 1,056                    
Acquisition date   Oct. 01, 2018                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Grammer Investment Holdings LLC Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:GrammerInvestmentHoldingsLLCMember bcsf:GrammerInvestmentHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Spread Above Index (as a percent) 10.00%   10.00% 10.00% 10.00% 10.00%            
PIK   10.00%         10.00% 10.00% 10.00% 10.00% 10.00% 10.00%
Interest rate 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00%
Principal/Shares $ 9 $ 8                    
Cost 791 790                    
Market Value $ 894 $ 830                    
Acquisition date   Oct. 01, 2018                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Grammer Investment Holdings LLC Warrants                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:GrammerInvestmentHoldingsLLCMember bcsf:GrammerInvestmentHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member] Warrant [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Principal/Shares $ 122 $ 122                    
Market Value $ 95 $ 126                    
Acquisition date Oct. 01, 2018 Oct. 01, 2018                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Grammer Purchaser, Inc First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Grammer Purchaser, Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member]   Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]            
Spread Above Index (as a percent) 4.50%   4.50% 4.50% 4.50% 4.50%            
Interest rate 7.57%   7.57% 7.57% 7.57% 7.57%            
Maturity date Sep. 30, 2024   Sep. 30, 2024 Sep. 30, 2024 Sep. 30, 2024 Sep. 30, 2024            
Principal/Shares $ 3,843                      
Cost 3,782                      
Market Value $ 3,834                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Grammer Purchaser, Inc First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Grammer Purchaser, Inc [Member] Grammer Purchaser, Inc [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Spread Above Index (as a percent)   4.50%         4.50% 4.50% 4.50% 4.50% 4.50% 4.50%
Interest rate   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Maturity date Sep. 30, 2024 Sep. 30, 2024 Sep. 30, 2024 Sep. 30, 2024 Sep. 30, 2024 Sep. 30, 2024 Sep. 30, 2024 Sep. 30, 2024 Sep. 30, 2024 Sep. 30, 2024 Sep. 30, 2024 Sep. 30, 2024
Principal/Shares   $ 7,319                    
Cost   7,202                    
Market Value $ (2) $ 7,319                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Great Expressions Dental Center PC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Great Expressions Dental Center P C [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Healthcare And Pharmaceuticals [Member]         Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Spread Above Index (as a percent)   4.75%         4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
PIK   (0.50%)         (0.50%) (0.50%) (0.50%) (0.50%) (0.50%) (0.50%)
Interest rate   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Maturity date   Sep. 28, 2023         Sep. 28, 2023 Sep. 28, 2023 Sep. 28, 2023 Sep. 28, 2023 Sep. 28, 2023 Sep. 28, 2023
Principal/Shares   $ 7,831                    
Cost   7,844                    
Market Value   $ 7,205                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Great Expressions Dental Center PC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Great Expressions Dental Center P C [Member]                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Healthcare And Pharmaceuticals [Member]         Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Spread Above Index (as a percent)   4.75%         4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
PIK   (0.50%)         (0.50%) (0.50%) (0.50%) (0.50%) (0.50%) (0.50%)
Interest rate   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Maturity date   Sep. 28, 2022         Sep. 28, 2022 Sep. 28, 2022 Sep. 28, 2022 Sep. 28, 2022 Sep. 28, 2022 Sep. 28, 2022
Principal/Shares   $ 1,027                    
Cost   1,025                    
Market Value   $ 929                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Great Expressions Dental Center PC First Lien Senior Secured Loan - Revolver Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Great Expressions Dental Center P C [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member]   Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]            
Spread Above Index (as a percent) 4.25%   4.25% 4.25% 4.25% 4.25%            
PIK (0.50%)   (0.50%) (0.50%) (0.50%) (0.50%)            
Interest rate 7.88%   7.88% 7.88% 7.88% 7.88%            
Maturity date Sep. 28, 2023   Sep. 28, 2023 Sep. 28, 2023 Sep. 28, 2023 Sep. 28, 2023            
Principal/Shares $ 986                      
Cost 983                      
Market Value $ 897                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Great Expressions Dental Center PC First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Great Expressions Dental Center P C [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member]   Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]            
Spread Above Index (as a percent) 4.25%   4.25% 4.25% 4.25% 4.25%            
PIK (0.50%)   (0.50%) (0.50%) (0.50%) (0.50%)            
Interest rate 7.88%   7.88% 7.88% 7.88% 7.88%            
Maturity date Sep. 28, 2023   Sep. 28, 2023 Sep. 28, 2023 Sep. 28, 2023 Sep. 28, 2023            
Principal/Shares $ 7,868                      
Cost 7,899                      
Market Value $ 7,278                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Green Street Parent, LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Green Street Parent, L L C [Member] Green Street Parent, L L C [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member]
Spread Above Index (as a percent) 5.50% 5.75% 5.50% 5.50% 5.50% 5.50% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate 8.62% 5.50% 8.62% 8.62% 8.62% 8.62% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Maturity date Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026
Principal/Shares $ 3,411 $ 14,190                    
Cost 3,369 13,988                    
Market Value $ 3,445 $ 14,190                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Green Street Parent, LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Green Street Parent, L L C [Member] Green Street Parent, L L C [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member]
Maturity date Aug. 27, 2025 Aug. 27, 2025 Aug. 27, 2025 Aug. 27, 2025 Aug. 27, 2025 Aug. 27, 2025 Aug. 27, 2025 Aug. 27, 2025 Aug. 27, 2025 Aug. 27, 2025 Aug. 27, 2025 Aug. 27, 2025
Cost $ (23) $ (29)                    
Market Value $ 24                      
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Green Street Parent, LLC First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Green Street Parent, L L C [Member] Green Street Parent, L L C [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] bcsf:FirstLienSeniorSecuredLoan2Member First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member
Investment, Industry Sector [Extensible Enumeration] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member] Banking [Member]
Spread Above Index (as a percent) 5.50% 5.00% 5.50% 5.50% 5.50% 5.50% 5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate 8.62% 5.50% 8.62% 8.62% 8.62% 8.62% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Maturity date Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026 Aug. 27, 2026
Principal/Shares $ 4,466 $ 4,500                    
Cost 4,390 4,411                    
Market Value $ 4,511 $ 4,500                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Horizon Telcom, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:HorizonTelcomIncMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Telecommunications [Member]         Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]
Spread Above Index (as a percent)   5.00%         5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Maturity date   Jun. 15, 2023         Jun. 15, 2023 Jun. 15, 2023 Jun. 15, 2023 Jun. 15, 2023 Jun. 15, 2023 Jun. 15, 2023
Principal/Shares   $ 13,104                    
Cost   13,045                    
Market Value   $ 13,104                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Horizon Telcom, Inc. First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:HorizonTelcomIncMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Telecommunications [Member]         Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]
Spread Above Index (as a percent)   5.00%         5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Maturity date   Jun. 15, 2023         Jun. 15, 2023 Jun. 15, 2023 Jun. 15, 2023 Jun. 15, 2023 Jun. 15, 2023 Jun. 15, 2023
Principal/Shares   $ 890                    
Cost   888                    
Market Value   $ 890                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Horizon Telcom, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:HorizonTelcomIncMember                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Telecommunications [Member]         Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]
Spread Above Index (as a percent)   5.00%         5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Maturity date   Jun. 15, 2023         Jun. 15, 2023 Jun. 15, 2023 Jun. 15, 2023 Jun. 15, 2023 Jun. 15, 2023 Jun. 15, 2023
Principal/Shares   $ 116                    
Cost   114                    
Market Value   $ 116                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Insigneo Financial Group LLC Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:InsigneoFinancialGroupLlcMember                      
Investment, Type [Extensible Enumeration] Common Stock [Member]   Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Finance [Member]   Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member]            
Cost $ 2,140                      
Market Value $ 2,135                      
Acquisition date Aug. 01, 2022                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: Insigneo Financial Group LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:InsigneoFinancialGroupLlcMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Finance [Member]   Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member]            
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 9.19%   9.19% 9.19% 9.19% 9.19%            
Maturity date Aug. 01, 2028   Aug. 01, 2028 Aug. 01, 2028 Aug. 01, 2028 Aug. 01, 2028            
Principal/Shares $ 3,825                      
Cost 3,731                      
Market Value $ 3,729                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: International Entertainment Investments Limited First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:InternationalEntertainmentInvestmentsLimitedMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Media: Diversified & Production   Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production            
Spread Above Index (as a percent) 4.75%   4.75% 4.75% 4.75% 4.75%            
Interest rate 6.47%   6.47% 6.47% 6.47% 6.47%            
Maturity date May 31, 2025   May 31, 2025 May 31, 2025 May 31, 2025 May 31, 2025            
Principal/Shares | £         £ 87              
Cost $ 107                      
Market Value $ 98                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember   bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: International Entertainment Investments Limited First Lien Senior Secured Loan Gbp libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:InternationalEntertainmentInvestmentsLimitedMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Media: Diversified & Production         Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production
Spread Above Index (as a percent)   4.75%         4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
Interest rate   5.06%         5.06% 5.06% 5.06% 5.06% 5.06% 5.06%
Maturity date   May 31, 2023         May 31, 2023 May 31, 2023 May 31, 2023 May 31, 2023 May 31, 2023 May 31, 2023
Principal/Shares | £                 £ 87      
Cost | £                 106      
Market Value | £                 £ 118      
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember         bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: International Senior Loan Program, LLC Equity Interest Investment Vehicles                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]
Investment, Issuer Name [Extensible Enumeration] International Senior Loan Program L L C [Member] International Senior Loan Program L L C [Member]                    
Investment, Type [Extensible Enumeration] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member] Equity Interest Investment Vehicles [Member]
Investment, Industry Sector [Extensible Enumeration] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member]
Principal/Shares $ 57,774 $ 41,823                    
Cost 54,892 39,596                    
Market Value $ 54,584 $ 44,444                    
Acquisition date Feb. 22, 2021 Feb. 22, 2021                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] false false false false false false false false false false false false
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: International Senior Loan Program, LLC Subordinated Note Investment Vehicles                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]
Investment, Issuer Name [Extensible Enumeration] International Senior Loan Program L L C [Member] International Senior Loan Program L L C [Member]                    
Investment, Type [Extensible Enumeration] bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember bcsf:SubordinatedNoteInvestmentVehiclesMember
Investment, Industry Sector [Extensible Enumeration] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member] Investment Vehicles [Member]
Spread Above Index (as a percent) 8.00% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00%
Interest rate 10.29% 9.00% 10.29% 10.29% 10.29% 10.29% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00%
Maturity date Feb. 22, 2028 Feb. 22, 2028 Feb. 22, 2028 Feb. 22, 2028 Feb. 22, 2028 Feb. 22, 2028 Feb. 22, 2028 Feb. 22, 2028 Feb. 22, 2028 Feb. 22, 2028 Feb. 22, 2028 Feb. 22, 2028
Principal/Shares $ 173,289 $ 125,437                    
Cost 173,289 125,437                    
Market Value $ 173,289 $ 125,437                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] false true false false false false true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Iris Holding, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:IrisHoldingInc.Member                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Containers Packaging And Glass [Member]   Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member] Containers Packaging And Glass [Member]            
Spread Above Index (as a percent) 4.75%   4.75% 4.75% 4.75% 4.75%            
Interest rate 7.89%   7.89% 7.89% 7.89% 7.89%            
Maturity date Jun. 15, 2028   Jun. 15, 2028 Jun. 15, 2028 Jun. 15, 2028 Jun. 15, 2028            
Principal/Shares $ 13,050                      
Cost 12,388                      
Market Value $ 12,032                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 0.50%   0.50% 0.50% 0.50% 0.50%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Island Medical Management Holdings, LLC First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:IslandMedicalManagementHoldingsLLCMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Healthcare And Pharmaceuticals [Member]         Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Spread Above Index (as a percent)   6.50%         6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Interest rate   7.50%         7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Maturity date   Sep. 01, 2023         Sep. 01, 2023 Sep. 01, 2023 Sep. 01, 2023 Sep. 01, 2023 Sep. 01, 2023 Sep. 01, 2023
Principal/Shares   $ 8,520                    
Cost   8,496                    
Market Value   $ 8,371                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: JHCC Holdings, LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:JHCCHoldingsLLCMember bcsf:JHCCHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member]
Spread Above Index (as a percent) 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate 9.42% 6.75% 9.42% 9.42% 9.42% 9.42% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025
Principal/Shares $ 21,318 $ 29,081                    
Cost 21,148 28,799                    
Market Value $ 20,679 $ 29,081                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: JHCC Holdings, LLC First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:JHCCHoldingsLLCMember bcsf:JHCCHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member]
Spread Above Index (as a percent) 5.75% 4.50% 5.75% 5.75% 5.75% 5.75% 4.50% 4.50% 4.50% 4.50% 4.50% 4.50%
Interest rate 9.42% 7.75% 9.42% 9.42% 9.42% 9.42% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75%
Maturity date   Sep. 09, 2025         Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025
Principal/Shares $ 8,353 $ 2,635                    
Cost 8,328 2,618                    
Market Value $ 8,103 $ 2,635                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] us-gaap:PrimeRateMember London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: JHCC Holdings, LLC First Lien Senior Secured Loan - Delayed Draw 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:JHCCHoldingsLLCMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Automotive Sector [Member]         Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date   Sep. 09, 2025         Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025
Principal/Shares   $ 5,782                    
Cost   5,776                    
Market Value   $ 5,782                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: JHCC Holdings, LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:JHCCHoldingsLLCMember bcsf:JHCCHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member] Automotive Sector [Member]
Spread Above Index (as a percent) 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate 9.42% 6.75% 9.42% 9.42% 9.42% 9.42% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025 Sep. 09, 2025
Principal/Shares $ 1,620 $ 894                    
Cost 1,592 863                    
Market Value $ 1,535 $ 894                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] us-gaap:PrimeRateMember London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember
Investment, Interest Rate, Floor 1.00% 2.00% 1.00% 1.00% 1.00% 1.00% 2.00% 2.00% 2.00% 2.00% 2.00% 2.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Jonathan Acquisition Company Second Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:JonathanAcquisitionCompanyMember bcsf:JonathanAcquisitionCompanyMember                    
Investment, Type [Extensible Enumeration] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member]
Spread Above Index (as a percent) 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00%
Interest rate 12.60% 10.00% 12.60% 12.60% 12.60% 12.60% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00%
Maturity date Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027
Principal/Shares $ 8,000 $ 8,000                    
Cost 7,836 7,821                    
Market Value $ 8,000 $ 8,000                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Kellstrom Aerospace Group, Inc Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:KellstromAerospaceGroupIncMember bcsf:KellstromAerospaceGroupIncMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Principal/Shares $ 1 $ 1                    
Cost 1,963 1,963                    
Market Value $ 792 $ 913                    
Acquisition date Jul. 01, 2019 Jul. 01, 2019                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Kellstrom Commercial Aerospace, Inc First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Kellstrom Commercial Aerospace, Inc [Member] Kellstrom Commercial Aerospace, Inc [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 6.25% 5.50% 6.25% 6.25% 6.25% 6.25% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
PIK (0.50%)   (0.50%) (0.50%) (0.50%) (0.50%)            
Interest rate 11.25% 6.50% 11.25% 11.25% 11.25% 11.25% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025
Principal/Shares $ 2,666 $ 2,239                    
Cost 2,626 2,176                    
Market Value $ 2,410 $ 1,919                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 0.00% 1.00% 1.00% 1.00% 1.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Kellstrom Commercial Aerospace, Inc First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Kellstrom Commercial Aerospace, Inc [Member] Kellstrom Commercial Aerospace, Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 6.00% 5.50% 6.00% 6.00% 6.00% 6.00% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 8.37% 6.50% 8.37% 8.37% 8.37% 8.37% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025 Jul. 01, 2025
Principal/Shares $ 30,082 $ 32,855                    
Cost 29,772 32,430                    
Market Value $ 28,277 $ 30,884                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Learning Pool First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:LearningPoolMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 3.00%   3.00% 3.00% 3.00% 3.00%            
PIK (4.25%)   (4.25%) (4.25%) (4.25%) (4.25%)            
Interest rate 9.18%   9.18% 9.18% 9.18% 9.18%            
Maturity date Jul. 07, 2028   Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028            
Principal/Shares | €       € 279                
Cost $ 360                      
Market Value $ 312                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Learning Pool First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:LearningPoolMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 3.00%   3.00% 3.00% 3.00% 3.00%            
PIK (4.25%)   (4.25%) (4.25%) (4.25%) (4.25%)            
Interest rate 9.18%   9.18% 9.18% 9.18% 9.18%            
Maturity date Jul. 07, 2028   Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028 Jul. 07, 2028            
Principal/Shares | £         £ 100              
Cost $ 129                      
Market Value $ 111                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Lightning Finco Limited First Lien Senior Secured Loan Euibor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:LightningFincoLimitedMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Media: Broadcasting & Subscription   Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription            
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 6.25%   6.25% 6.25% 6.25% 6.25%            
Maturity date Sep. 01, 2028   Sep. 01, 2028 Sep. 01, 2028 Sep. 01, 2028 Sep. 01, 2028            
Principal/Shares $ 1,300                      
Cost 1,417                      
Market Value $ 1,274                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Lightning Finco Limited First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:LightningFincoLimitedMember bcsf:LightningFincoLimitedMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription
Spread Above Index (as a percent) 5.50% 5.75% 5.50% 5.50% 5.50% 5.50% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate 8.57% 6.50% 8.57% 8.57% 8.57% 8.57% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Sep. 01, 2028 Jul. 14, 2028 Sep. 01, 2028 Sep. 01, 2028 Sep. 01, 2028 Sep. 01, 2028 Jul. 14, 2028 Jul. 14, 2028 Jul. 14, 2028 Jul. 14, 2028 Jul. 14, 2028 Jul. 14, 2028
Principal/Shares $ 1,443 $ 4,350                    
Cost 1,408 4,234                    
Market Value $ 1,443 $ 4,350                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Lightning Finco Limited First Lien Senior Secured Loan Libor 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:LightningFincoLimitedMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Media: Broadcasting & Subscription         Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription Media: Broadcasting & Subscription
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   6.50%         6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date   Jul. 14, 2028         Jul. 14, 2028 Jul. 14, 2028 Jul. 14, 2028 Jul. 14, 2028 Jul. 14, 2028 Jul. 14, 2028
Principal/Shares   $ 4,629                    
Cost   4,506                    
Market Value   $ 4,629                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   0.75%         0.75% 0.75% 0.75% 0.75% 0.75% 0.75%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Lightning Holdings B, LLC Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member] Investment, Affiliated Issuer, Controlled [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:LightningHoldingsBLLCMember bcsf:LightningHoldingsBLLCMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo Controlled Affiliate [Member] Transportation Cargo Controlled Affiliate [Member] Transportation Cargo Controlled Affiliate [Member] Transportation Cargo Controlled Affiliate [Member] Transportation Cargo Controlled Affiliate [Member] Transportation Cargo Controlled Affiliate [Member] Transportation Cargo Controlled Affiliate [Member] Transportation Cargo Controlled Affiliate [Member] Transportation Cargo Controlled Affiliate [Member] Transportation Cargo Controlled Affiliate [Member] Transportation Cargo Controlled Affiliate [Member] Transportation Cargo Controlled Affiliate [Member]
Principal/Shares $ 23,355 $ 13,843                    
Cost 23,665 14,152                    
Market Value $ 25,101 $ 14,851                    
Acquisition date Jan. 02, 2020 Jan. 02, 2020                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: MRH Trowe Beteiligungsgesellschaft MBH First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Margaux U K Finance Limited [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member]   Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]            
Spread Above Index (as a percent) 6.50%   6.50% 6.50% 6.50% 6.50%            
Interest rate 6.50%   6.50% 6.50% 6.50% 6.50%            
Maturity date Jul. 26, 2028   Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028            
Principal/Shares | £         £ 1,865              
Cost $ 1,858                      
Market Value $ 1,828                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: MRHT Facility A First Lien Senior Secured Loan Euibor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:MRHTFacilityAMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Fire Insurance [Member]         Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]
Spread Above Index (as a percent)   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Maturity date   Jul. 26, 2028         Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028
Principal/Shares | €               € 216        
Cost | €               248        
Market Value | €               € 245        
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:EuroInterbankOfferRateMember         bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember
Investment, Interest Rate, Floor   0.00%         0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: MRHT First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:MRHTMember bcsf:MRHTAcquisitionFacilityMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 5.50%   5.50% 5.50% 5.50% 5.50%            
Maturity date Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028
Principal/Shares | €       € 500                
Cost $ 535                      
Cost | €               € (6)        
Market Value $ 485                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: MRHT First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:MRHTMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member]   Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]            
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 5.70%   5.70% 5.70% 5.70% 5.70%            
Maturity date Jul. 26, 2028   Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028            
Principal/Shares | €       € 216                
Cost $ 249                      
Market Value $ 209                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: MRHT First Lien Senior Secured Loan 3                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:MRHTMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member]   Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]            
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 5.50%   5.50% 5.50% 5.50% 5.50%            
Maturity date Jul. 26, 2028   Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028 Jul. 26, 2028            
Principal/Shares | €       € 10,000                
Cost $ 10,125                      
Market Value $ 9,704                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: MRI Software LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:MRISoftwareLLCMember bcsf:MRISoftwareLLCMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 9.17% 6.50% 9.17% 9.17% 9.17% 9.17% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026
Principal/Shares $ 25,728 $ 25,926                    
Cost 25,667 25,850                    
Market Value $ 25,004 $ 25,926                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: MRI Software LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:MRISoftwareLLCMember bcsf:MRISoftwareLLCMember                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Maturity date Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026 Feb. 10, 2026
Cost $ 52 $ 48                    
Market Value $ (50)                      
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: MZR Aggregator Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:MZRAggregatorMember bcsf:MZRAggregatorMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member]
Principal/Shares $ 1 $ 1                    
Cost 798 798                    
Market Value $ 791 $ 798                    
Acquisition date Dec. 22, 2020 Dec. 22, 2020                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: MZR Buyer, LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:MZRBuyerLLCMember bcsf:MZRBuyerLLCMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member]
Spread Above Index (as a percent) 6.75% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Interest rate 10.42% 7.75% 10.42% 10.42% 10.42% 10.42% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75%
Maturity date Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026
Principal/Shares $ 16,849 $ 40,228                    
Cost 16,651 39,551                    
Market Value $ 16,849 $ 40,228                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: MZR Buyer, LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:MZRBuyerLLCMember bcsf:MZRBuyerLLCMember                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member]
Maturity date Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026 Dec. 21, 2026
Cost $ (73) $ (86)                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Mach Acquisition R/C Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Mach Acquisition R C [Member] Mach Acquisition R C [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 7.50%   7.50% 7.50% 7.50% 7.50%            
Interest rate 10.11%   10.11% 10.11% 10.11% 10.11%            
Maturity date Oct. 18, 2026 Oct. 18, 2026 Oct. 18, 2026 Oct. 18, 2026 Oct. 18, 2026 Oct. 18, 2026 Oct. 18, 2026 Oct. 18, 2026 Oct. 18, 2026 Oct. 18, 2026 Oct. 18, 2026 Oct. 18, 2026
Principal/Shares $ 3,013                      
Cost 2,850                      
Cost   $ (193)                    
Market Value $ 2,410 $ (201)                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Mach Acquisition T/L First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:MachAcquisitionTLMember bcsf:MachAcquisitionTLMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Interest rate 10.24% 8.50% 10.24% 10.24% 10.24% 10.24% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50%
Maturity date Oct. 18, 2026   Oct. 18, 2026 Oct. 18, 2026 Oct. 18, 2026 Oct. 18, 2026            
Principal/Shares $ 33,143 $ 32,640                    
Cost 32,602 32,006                    
Market Value $ 31,154 $ 31,987                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Margaux Acquisition Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Margaux Acquisition Inc [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Fire Insurance [Member]         Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]
Spread Above Index (as a percent)   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate   6.50%         6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date   Dec. 19, 2024         Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024
Principal/Shares   $ 28,334                    
Cost   28,000                    
Market Value   $ 28,334                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Margaux Acquisition Inc. First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Margaux Acquisition Inc [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Fire Insurance [Member]         Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]
Spread Above Index (as a percent)   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate   6.50%         6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date   Dec. 19, 2024         Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024
Principal/Shares   $ 9,198                    
Cost   9,173                    
Market Value   $ 9,198                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Margaux Acquisition Inc. First Lien Senior Secured Loan - Delayed Draw Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Margaux Acquisition Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member]   Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 8.03%   8.03% 8.03% 8.03% 8.03%            
Maturity date Dec. 19, 2024   Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024            
Principal/Shares $ 9,128                      
Cost 9,109                      
Market Value $ 9,083                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Margaux Acquisition Inc. First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Margaux Acquisition Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member]   Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 8.03%   8.03% 8.03% 8.03% 8.03%            
Maturity date Dec. 19, 2024   Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024            
Principal/Shares $ 17,637                      
Cost 17,472                      
Market Value $ 17,548                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Margaux Acquisition, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Margaux Acquisition Inc [Member] Margaux Acquisition Inc [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]
Maturity date Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024
Cost $ (21) $ (28)                    
Market Value $ (14)                      
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Margaux UK Finance Limited First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Margaux U K Finance Limited [Member] Margaux U K Finance Limited [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]
Spread Above Index (as a percent) 5.75% 5.50% 5.75% 5.75% 5.75% 5.75% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 7.06% 6.50% 7.06% 7.06% 7.06% 7.06% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024
Principal/Shares $ 7,512               £ 7,551      
Cost 9,697               9,740      
Market Value $ 8,307               £ 10,218      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember us-gaap:LondonInterbankOfferedRateLiborSwapRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember us-gaap:LondonInterbankOfferedRateLiborSwapRateMember us-gaap:LondonInterbankOfferedRateLiborSwapRateMember us-gaap:LondonInterbankOfferedRateLiborSwapRateMember us-gaap:LondonInterbankOfferedRateLiborSwapRateMember us-gaap:LondonInterbankOfferedRateLiborSwapRateMember us-gaap:LondonInterbankOfferedRateLiborSwapRateMember
Investment, Interest Rate, Floor 0.00% 1.00% 0.00% 0.00% 0.00% 0.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Margaux UK Finance Limited First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Margaux U K Finance Limited [Member] Margaux U K Finance Limited [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]
Spread Above Index (as a percent)   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate   6.50%         6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024 Dec. 19, 2024
Principal/Shares | £                 £ 89      
Cost | £                 112      
Cost $ (6)                      
Market Value $ (6)               £ 120      
Investment, Variable Interest Rate, Type [Extensible Enumeration]   us-gaap:LondonInterbankOfferedRateLiborSwapRateMember         us-gaap:LondonInterbankOfferedRateLiborSwapRateMember us-gaap:LondonInterbankOfferedRateLiborSwapRateMember us-gaap:LondonInterbankOfferedRateLiborSwapRateMember us-gaap:LondonInterbankOfferedRateLiborSwapRateMember us-gaap:LondonInterbankOfferedRateLiborSwapRateMember us-gaap:LondonInterbankOfferedRateLiborSwapRateMember
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Meriplex Communications, Ltd. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Meriplex Communications Ltd. [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Telecommunications [Member]   Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]            
Spread Above Index (as a percent) 5.00%   5.00% 5.00% 5.00% 5.00%            
Interest rate 7.95%   7.95% 7.95% 7.95% 7.95%            
Maturity date Jul. 17, 2028   Jul. 17, 2028 Jul. 17, 2028 Jul. 17, 2028 Jul. 17, 2028            
Principal/Shares $ 15,294                      
Cost 15,009                      
Market Value $ 14,988                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Meriplex Communications, Ltd. First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Meriplex Communications Ltd. [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Telecommunications [Member]   Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]            
Spread Above Index (as a percent) 5.00%   5.00% 5.00% 5.00% 5.00%            
Interest rate 7.95%   7.95% 7.95% 7.95% 7.95%            
Maturity date Jul. 17, 2028   Jul. 17, 2028 Jul. 17, 2028 Jul. 17, 2028 Jul. 17, 2028            
Principal/Shares $ 2,353                      
Cost 2,234                      
Market Value $ 2,231                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Meriplex Communications, Ltd. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Meriplex Communications Ltd. [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Telecommunications [Member]   Telecommunications [Member] Telecommunications [Member] Telecommunications [Member] Telecommunications [Member]            
Spread Above Index (as a percent) 5.00%   5.00% 5.00% 5.00% 5.00%            
Interest rate 8.18%   8.18% 8.18% 8.18% 8.18%            
Maturity date Jul. 17, 2028   Jul. 17, 2028 Jul. 17, 2028 Jul. 17, 2028 Jul. 17, 2028            
Principal/Shares $ 188                      
Cost 134                      
Market Value $ 132                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Mertus 522. GmbH First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:Mertus522GmbHMember bcsf:Mertus522GmbHMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Spread Above Index (as a percent) 6.25% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Interest rate 8.11% 6.25% 8.11% 8.11% 8.11% 8.11% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026
Principal/Shares | €       € 131       € 225        
Cost $ 142             247        
Market Value $ 126             € 255        
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember
Investment, Interest Rate, Floor 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Mertus 522. GmbH First Lien Senior Secured Loan - Delayed Draw Euibor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:Mertus522GmbHMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Healthcare And Pharmaceuticals [Member]         Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Spread Above Index (as a percent)   6.25%         6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Interest rate   6.25%         6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date   May 28, 2026         May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026
Principal/Shares | €               € 131        
Cost | €               142        
Market Value | €               € 149        
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:EuroInterbankOfferRateMember         bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember
Investment, Interest Rate, Floor   0.00%         0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Mertus 522. GmbH First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:Mertus522GmbHMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member]   Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]            
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 6.25%   6.25% 6.25% 6.25% 6.25%            
Maturity date May 28, 2026   May 28, 2026 May 28, 2026 May 28, 2026 May 28, 2026            
Principal/Shares | €       € 225                
Cost $ 247                      
Market Value $ 216                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Morrow Sodali First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Morrow Sodali [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Banking Finance Insurance And Real Estate Sector [Member]   Banking Finance Insurance And Real Estate Sector [Member] Banking Finance Insurance And Real Estate Sector [Member] Banking Finance Insurance And Real Estate Sector [Member] Banking Finance Insurance And Real Estate Sector [Member]            
Maturity date Apr. 25, 2028   Apr. 25, 2028 Apr. 25, 2028 Apr. 25, 2028 Apr. 25, 2028            
Cost $ (19)                      
Market Value $ (40)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Morrow Sodali First Lien Senior Secured Loan - Delayed Draw 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Morrow Sodali [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Banking Finance Insurance And Real Estate Sector [Member]   Banking Finance Insurance And Real Estate Sector [Member] Banking Finance Insurance And Real Estate Sector [Member] Banking Finance Insurance And Real Estate Sector [Member] Banking Finance Insurance And Real Estate Sector [Member]            
Maturity date Apr. 25, 2028   Apr. 25, 2028 Apr. 25, 2028 Apr. 25, 2028 Apr. 25, 2028            
Cost $ (67)                      
Market Value $ (34)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Morrow Sodali First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Morrow Sodali [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Banking Finance Insurance And Real Estate Sector [Member]   Banking Finance Insurance And Real Estate Sector [Member] Banking Finance Insurance And Real Estate Sector [Member] Banking Finance Insurance And Real Estate Sector [Member] Banking Finance Insurance And Real Estate Sector [Member]            
Spread Above Index (as a percent) 5.00%   5.00% 5.00% 5.00% 5.00%            
Interest rate 8.65%   8.65% 8.65% 8.65% 8.65%            
Maturity date Apr. 25, 2028   Apr. 25, 2028 Apr. 25, 2028 Apr. 25, 2028 Apr. 25, 2028            
Principal/Shares $ 266                      
Cost 236                      
Market Value $ 234                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Music Creation Group Bidco GmbH First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:MusicCreationGroupBidcoGmbHMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Media: Diversified & Production   Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production Media: Diversified & Production            
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 9.39%   9.39% 9.39% 9.39% 9.39%            
Maturity date Aug. 03, 2027   Aug. 03, 2027 Aug. 03, 2027 Aug. 03, 2027 Aug. 03, 2027            
Principal/Shares $ 4,065                      
Cost 3,974                      
Market Value $ 4,014                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.25%   0.25% 0.25% 0.25% 0.25%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: NPC International, Inc. Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:NPCInternationalIncMember bcsf:NPCInternationalIncMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Beverage Foodand Tobacco Sector [Member] Beverage Foodand Tobacco Sector [Member] Beverage Foodand Tobacco Sector [Member] Beverage Foodand Tobacco Sector [Member] Beverage Foodand Tobacco Sector [Member] Beverage Foodand Tobacco Sector [Member] Beverage Foodand Tobacco Sector [Member] Beverage Foodand Tobacco Sector [Member] Beverage Foodand Tobacco Sector [Member] Beverage Foodand Tobacco Sector [Member] Beverage Foodand Tobacco Sector [Member] Beverage Foodand Tobacco Sector [Member]
Principal/Shares $ 428 $ 564                    
Cost 639 843                    
Market Value $ 121 $ 228                    
Acquisition date Apr. 01, 2021 Apr. 01, 2021                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true false true true true true false false false false false false
Investment, Identifier [Axis]: New Era Cap Co., Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:NewEraCapCoIncMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Consumer Goods Non Durable Sector [Member]         Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]
Spread Above Index (as a percent)   6.50%         6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Interest rate   7.50%         7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Maturity date   Sep. 10, 2023         Sep. 10, 2023 Sep. 10, 2023 Sep. 10, 2023 Sep. 10, 2023 Sep. 10, 2023 Sep. 10, 2023
Principal/Shares   $ 9,970                    
Cost   9,970                    
Market Value   $ 9,970                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: New Look (Delaware) Corporation First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] New Look (Delaware) Corporation [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Retail Sector [Member]   Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]            
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 9.17%   9.17% 9.17% 9.17% 9.17%            
Maturity date May 26, 2028   May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028            
Principal/Shares $ 9,677                      
Cost 9,590                      
Market Value $ 9,580                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: New Look (Delaware) Corporation First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] New Look (Delaware) Corporation [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Retail Sector [Member]   Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]            
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 9.67%   9.67% 9.67% 9.67% 9.67%            
Maturity date May 26, 2028   May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028            
Principal/Shares $ 386                      
Cost 376                      
Market Value $ 363                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: New Look Vision Group First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] New Look Vision Group [Member] New Look Vision Group [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]
Spread Above Index (as a percent) 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 9.67% 6.25% 9.67% 9.67% 9.67% 9.67% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028
Principal/Shares           $ 81       $ 322    
Cost $ 64                 310    
Market Value $ 58                 $ 322    
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: New Look Vision Group First Lien Senior Secured Loan - Delayed Draw Cdor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   New Look Vision Group [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Retail Sector [Member]         Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]
Spread Above Index (as a percent)   5.25%         5.25% 5.25% 5.25% 5.25% 5.25% 5.25%
Interest rate   6.25%         6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date   May 26, 2028         May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028
Principal/Shares                   $ 2,380    
Cost                   1,868    
Market Value                   $ 1,883    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:CanadianDollarOfferedRateMember         bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: New Look Vision Group First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] New Look Vision Group [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Retail Sector [Member]   Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]            
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 9.67%   9.67% 9.67% 9.67% 9.67%            
Maturity date May 26, 2026   May 26, 2026 May 26, 2026 May 26, 2026 May 26, 2026            
Principal/Shares           $ 1,419            
Cost $ 1,055                      
Market Value $ 1,009                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:CanadianDollarOfferedRateMember   bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: New Look Vision Group First Lien Senior Secured Loan - Revolver Cdor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   New Look Vision Group [Member]                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Retail Sector [Member]         Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]
Spread Above Index (as a percent)   5.25%         5.25% 5.25% 5.25% 5.25% 5.25% 5.25%
Interest rate   6.25%         6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date   May 26, 2026         May 26, 2026 May 26, 2026 May 26, 2026 May 26, 2026 May 26, 2026 May 26, 2026
Principal/Shares                   $ 313    
Cost                   228    
Market Value                   $ 248    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:CanadianDollarOfferedRateMember         bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: New Look Vision Group First Lien Senior Secured Loan Cdor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   New Look Vision Group [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Retail Sector [Member]         Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]
Spread Above Index (as a percent)   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate   6.25%         6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date   May 26, 2028         May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028 May 26, 2028
Principal/Shares                   $ 9,750    
Cost                   9,653    
Market Value                   $ 9,750    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:CanadianDollarOfferedRateMember         bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: New Milani Group LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:NewMilaniGroupLLCMember bcsf:NewMilaniGroupLLCMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member]
Spread Above Index (as a percent) 6.00% 6.50% 6.00% 6.00% 6.00% 6.00% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Interest rate 9.67% 7.50% 9.67% 9.67% 9.67% 9.67% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Maturity date   Jun. 06, 2024         Jun. 06, 2024 Jun. 06, 2024 Jun. 06, 2024 Jun. 06, 2024 Jun. 06, 2024 Jun. 06, 2024
Principal/Shares $ 21,531 $ 16,752                    
Cost 21,039 16,678                    
Market Value $ 21,100 $ 16,250                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: OGH Bidco Limited First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] OGH Bidco Limited [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Media Publishing [Member]   Media Publishing [Member] Media Publishing [Member] Media Publishing [Member] Media Publishing [Member]            
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 8.53%   8.53% 8.53% 8.53% 8.53%            
Maturity date Jun. 29, 2029   Jun. 29, 2029 Jun. 29, 2029 Jun. 29, 2029 Jun. 29, 2029            
Principal/Shares | £         £ 13,299              
Cost $ 15,685                      
Market Value $ 14,706                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember   bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: OGH Bidco Limited First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] OGH Bidco Limited [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Media Publishing [Member]   Media Publishing [Member] Media Publishing [Member] Media Publishing [Member] Media Publishing [Member]            
Maturity date Jun. 29, 2029   Jun. 29, 2029 Jun. 29, 2029 Jun. 29, 2029 Jun. 29, 2029            
Cost $ (71)                      
Market Value $ (69)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: OGH Bidco Limited First Lien Senior Secured Loan - Delayed Draw SONIA                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] OGH Bidco Limited [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Media Publishing [Member]   Media Publishing [Member] Media Publishing [Member] Media Publishing [Member] Media Publishing [Member]            
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 7.44%   7.44% 7.44% 7.44% 7.44%            
Maturity date Jun. 29, 2029   Jun. 29, 2029 Jun. 29, 2029 Jun. 29, 2029 Jun. 29, 2029            
Principal/Shares | £         £ 5,172              
Cost $ 5,893                      
Market Value $ 5,719                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember   bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Omni Intermediate DD T/L First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:OmniIntermediateDDTL1Member                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Transportation Cargo [Member]         Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Spread Above Index (as a percent)   5.00%         5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Maturity date   Nov. 23, 2026         Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026
Principal/Shares   $ 776                    
Cost   769                    
Market Value   $ 768                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Omni Intermediate DD T/L First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:OmniIntermediateDDTL2Member                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Transportation Cargo [Member]         Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Spread Above Index (as a percent)   5.00%         5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Maturity date   Nov. 23, 2026         Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026
Principal/Shares   $ 46                    
Cost   37                    
Market Value   $ 37                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Omni Intermediate First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:OmniIntermediateMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member]   Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]            
Spread Above Index (as a percent) 5.00%   5.00% 5.00% 5.00% 5.00%            
Interest rate 8.69%   8.69% 8.69% 8.69% 8.69%            
Maturity date Nov. 23, 2026   Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026            
Principal/Shares $ 814                      
Cost 815                      
Market Value $ 807                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Omni Intermediate First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:OmniIntermediateMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member]   Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]            
Spread Above Index (as a percent) 5.00%   5.00% 5.00% 5.00% 5.00%            
Interest rate 6.00%   6.00% 6.00% 6.00% 6.00%            
Maturity date Nov. 23, 2026   Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026            
Principal/Shares $ 366                      
Cost 359                      
Market Value $ 366                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Omni Intermediate First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:OmniIntermediateMember                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member]   Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]            
Maturity date Nov. 30, 2026   Nov. 30, 2026 Nov. 30, 2026 Nov. 30, 2026 Nov. 30, 2026            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Omni Intermediate Holdings Closing Date Term Loan First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:OmniIntermediateHoldingsClosingDateTermLoanMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Transportation Cargo [Member]         Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Spread Above Index (as a percent)   5.00%         5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Maturity date   Nov. 23, 2026         Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026 Nov. 23, 2026
Principal/Shares   $ 7,306                    
Cost   7,233                    
Market Value   $ 7,233                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Omni Intermediate R/C First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Omni Intermediate R C [Member]                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Transportation Cargo [Member]         Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Spread Above Index (as a percent)   5.00%         5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Maturity date   Nov. 23, 2025         Nov. 23, 2025 Nov. 23, 2025 Nov. 23, 2025 Nov. 23, 2025 Nov. 23, 2025 Nov. 23, 2025
Principal/Shares   $ 183                    
Cost   183                    
Market Value   $ 176                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Omni Logistics, LLC Second Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:OmniLogisticsLLCMember bcsf:OmniLogisticsLLCMember                    
Investment, Type [Extensible Enumeration] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Spread Above Index (as a percent) 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00%
Interest rate 12.69% 10.00% 12.69% 12.69% 12.69% 12.69% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00%
Maturity date Dec. 30, 2027 Dec. 30, 2027 Dec. 30, 2027 Dec. 30, 2027 Dec. 30, 2027 Dec. 30, 2027 Dec. 30, 2027 Dec. 30, 2027 Dec. 30, 2027 Dec. 30, 2027 Dec. 30, 2027 Dec. 30, 2027
Principal/Shares $ 8,770 $ 13,770                    
Cost 8,668 13,527                    
Market Value $ 8,770 $ 13,770                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Opus 2 First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:Opus2Member                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Maturity date   May 05, 2028         May 05, 2028 May 05, 2028 May 05, 2028 May 05, 2028 May 05, 2028 May 05, 2028
Cost | £                 £ (173)      
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Opus 2 First Lien Senior Secured Loan Sonia                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:Opus2Member                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate   5.55%         5.55% 5.55% 5.55% 5.55% 5.55% 5.55%
Maturity date   May 05, 2028         May 05, 2028 May 05, 2028 May 05, 2028 May 05, 2028 May 05, 2028 May 05, 2028
Principal/Shares | £                 £ 123      
Cost | £                 167      
Market Value | £                 £ 166      
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:SterlingOvernightInterbankAverageRateMember         bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Opus2 Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:Opus2Member bcsf:Opus2Member                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Principal/Shares $ 2,272 $ 1,460                    
Cost 2,900 1,769                    
Market Value $ 2,880 $ 2,373                    
Acquisition date Jun. 16, 2021 Jun. 16, 2021                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false true false false false false true true true true true true
Investment, Identifier [Axis]: Opus2 First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:Opus2Member                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 5.28%   5.28% 5.28% 5.28% 5.28%            
Interest rate 5.97%   5.97% 5.97% 5.97% 5.97%            
Maturity date May 05, 2028   May 05, 2028 May 05, 2028 May 05, 2028 May 05, 2028            
Principal/Shares $ 123                      
Cost 167                      
Market Value $ 137                      
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: PPX Class A Units Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:PPXClassAUnitsMember                    
Investment, Type [Extensible Enumeration]   Preferred Stock [Member]         Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration]   Hospitality Holdings [Member]         Hospitality Holdings [Member] Hospitality Holdings [Member] Hospitality Holdings [Member] Hospitality Holdings [Member] Hospitality Holdings [Member] Hospitality Holdings [Member]
Principal/Shares   $ 33                    
Market Value   $ 163                    
Acquisition date   Jul. 29, 2021                    
Investment, Restriction Status [true false]   true         true true true true true true
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   true         true true true true true true
Investment, Identifier [Axis]: PPX Class B Units Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:PPXClassBUnitsMember                    
Investment, Type [Extensible Enumeration]   Preferred Stock [Member]         Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration]   Hospitality Holdings [Member]         Hospitality Holdings [Member] Hospitality Holdings [Member] Hospitality Holdings [Member] Hospitality Holdings [Member] Hospitality Holdings [Member] Hospitality Holdings [Member]
Principal/Shares   $ 33                    
Cost   5,000                    
Market Value   $ 5,279                    
Acquisition date   Jul. 29, 2021                    
Investment, Restriction Status [true false]   true         true true true true true true
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   true         true true true true true true
Investment, Identifier [Axis]: PPX Preferred Equity 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:PPXMember                      
Investment, Type [Extensible Enumeration] Preferred Stock [Member]   Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Hospitality Holdings [Member]   Hospitality Holdings [Member] Hospitality Holdings [Member] Hospitality Holdings [Member] Hospitality Holdings [Member]            
Principal/Shares $ 33                      
Market Value $ 163                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: PPX Preferred Equity 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:PPXMember                      
Investment, Type [Extensible Enumeration] Preferred Stock [Member]   Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Hospitality Holdings [Member]   Hospitality Holdings [Member] Hospitality Holdings [Member] Hospitality Holdings [Member] Hospitality Holdings [Member]            
Principal/Shares $ 33                      
Cost 5,000                      
Market Value $ 5,678                      
Acquisition date Jul. 29, 2021                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: Paisley Bidco Limited First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Paisley Bidco Limited [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member]   Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]            
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 6.93%   6.93% 6.93% 6.93% 6.93%            
Maturity date Nov. 26, 2028   Nov. 26, 2028 Nov. 26, 2028 Nov. 26, 2028 Nov. 26, 2028            
Principal/Shares | €       € 3,676                
Cost $ 4,336                      
Market Value $ 4,106                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Paisley Bidco Limited First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Paisley Bidco Limited [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Fire Insurance [Member]         Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]
Maturity date   Nov. 24, 2028         Nov. 24, 2028 Nov. 24, 2028 Nov. 24, 2028 Nov. 24, 2028 Nov. 24, 2028 Nov. 24, 2028
Cost | €               € (84)        
Market Value | €               € (86)        
Investment, Interest Rate, Floor   0.00%         0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Paisley Bidco Limited First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Paisley Bidco Limited [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member]   Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]            
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 6.19%   6.19% 6.19% 6.19% 6.19%            
Maturity date Nov. 26, 2028   Nov. 26, 2028 Nov. 26, 2028 Nov. 26, 2028 Nov. 26, 2028            
Principal/Shares | £         £ 32              
Cost $ 36                      
Market Value $ 31                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Paisley Bidco Limited First Lien Senior Secured Loan Euibor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Paisley Bidco Limited [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Fire Insurance [Member]         Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]
Spread Above Index (as a percent)   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Maturity date   Nov. 24, 2028         Nov. 24, 2028 Nov. 24, 2028 Nov. 24, 2028 Nov. 24, 2028 Nov. 24, 2028 Nov. 24, 2028
Principal/Shares | £                 £ 3,210      
Cost | £                 3,583      
Market Value | £                 £ 3,614      
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:EuroInterbankOfferRateMember         bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember
Investment, Interest Rate, Floor   0.00%         0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Parcel 2 Go Acquisition Facility First Lien Senior Secured Loan Sonia                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Parcel2 Go Acquisition Facility [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   5.92%         5.92% 5.92% 5.92% 5.92% 5.92% 5.92%
Maturity date   Jul. 15, 2028         Jul. 15, 2028 Jul. 15, 2028 Jul. 15, 2028 Jul. 15, 2028 Jul. 15, 2028 Jul. 15, 2028
Principal/Shares | £                 £ 3,863      
Cost | £                 4,982      
Market Value | £                 £ 5,183      
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:SterlingOvernightInterbankAverageRateMember         bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Parcel 2 Go Facility B First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:Parcel2GoFacilityBMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   5.80%         5.80% 5.80% 5.80% 5.80% 5.80% 5.80%
Maturity date   Jul. 15, 2028         Jul. 15, 2028 Jul. 15, 2028 Jul. 15, 2028 Jul. 15, 2028 Jul. 15, 2028 Jul. 15, 2028
Principal/Shares | £                 £ 125      
Cost | £                 169      
Market Value | £                 £ 169      
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:SterlingOvernightInterbankAverageRateMember         bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Parcel 2 Go Shares Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:Parcel2GoSharesMember                    
Investment, Type [Extensible Enumeration]   Common Stock [Member]         Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Principal/Shares   $ 2,881                    
Cost   3,983                    
Market Value   $ 3,899                    
Investment, Restriction Status [true false]   true         true true true true true true
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   true         true true true true true true
Investment, Identifier [Axis]: Parcel2Go Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:Parcel2GoMember                      
Investment, Type [Extensible Enumeration] Common Stock [Member]   Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Principal/Shares | €       € 3,605                
Cost $ 4,237                      
Market Value $ 3,462                      
Acquisition date Jul. 15, 2021 Jul. 15, 2021                    
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: Parcel2Go First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:Parcel2GoMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 7.44%   7.44% 7.44% 7.44% 7.44%            
Maturity date Jul. 15, 2028   Jul. 15, 2028 Jul. 15, 2028 Jul. 15, 2028 Jul. 15, 2028            
Principal/Shares $ 39                      
Cost 50                      
Market Value $ 43                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Parcel2Go First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:Parcel2GoMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 7.94%   7.94% 7.94% 7.94% 7.94%            
Maturity date Jul. 15, 2028   Jul. 15, 2028 Jul. 15, 2028 Jul. 15, 2028 Jul. 15, 2028            
Principal/Shares | £         £ 125              
Cost $ 169                      
Market Value $ 140                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember   bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Parmenion First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ParmenionMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Finance [Member]   Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 6.69%   6.69% 6.69% 6.69% 6.69%            
Maturity date May 11, 2029   May 11, 2029 May 11, 2029 May 11, 2029 May 11, 2029            
Principal/Shares | £         £ 328              
Cost $ 408                      
Market Value $ 366                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember   bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: PetroChoice Holdings, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:PetroChoiceHoldingsIncMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Wholesale [Member]         Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member]
Spread Above Index (as a percent)   5.00%         5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Maturity date   Aug. 19, 2022         Aug. 19, 2022 Aug. 19, 2022 Aug. 19, 2022 Aug. 19, 2022 Aug. 19, 2022 Aug. 19, 2022
Principal/Shares   $ 9,740                    
Cost   9,721                    
Market Value   $ 9,327                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   false         false false false false false false
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: PetroChoice Holdings, Inc. First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:PetroChoiceHoldingsIncMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Wholesale [Member]         Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member] Wholesale [Member]
Spread Above Index (as a percent)   5.00%         5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Maturity date   Aug. 19, 2022         Aug. 19, 2022 Aug. 19, 2022 Aug. 19, 2022 Aug. 19, 2022 Aug. 19, 2022 Aug. 19, 2022
Principal/Shares   $ 6,445                    
Cost   6,412                    
Market Value   $ 6,171                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   false         false false false false false false
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Precision Ultimate Holdings, LLC Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:PrecisionUltimateHoldingsLLCMember bcsf:PrecisionUltimateHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Principal/Shares $ 1,417 $ 1,417                    
Cost 1,417 1,417                    
Market Value $ 1,401 $ 1,204                    
Acquisition date Nov. 06, 2019 Nov. 06, 2019                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Premier Imaging, LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Premier Imaging, L L C [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member]   Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 8.87%   8.87% 8.87% 8.87% 8.87%            
Maturity date Jan. 02, 2025   Jan. 02, 2025 Jan. 02, 2025 Jan. 02, 2025 Jan. 02, 2025            
Principal/Shares $ 7,159                      
Cost 7,074                      
Market Value $ 7,087                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Premier Imaging, LLC First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Premier Imaging, L L C [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member]   Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 8.87%   8.87% 8.87% 8.87% 8.87%            
Maturity date Jan. 02, 2025   Jan. 02, 2025 Jan. 02, 2025 Jan. 02, 2025 Jan. 02, 2025            
Principal/Shares $ 1,941                      
Cost 1,863                      
Market Value $ 1,873                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Quidditch Acquisition, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:QuidditchAcquisitionIncMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Hotel, Gaming And Leisure [Member]   Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member]            
Spread Above Index (as a percent) 7.00%   7.00% 7.00% 7.00% 7.00%            
Interest rate 9.52%   9.52% 9.52% 9.52% 9.52%            
Maturity date Mar. 21, 2025   Mar. 21, 2025 Mar. 21, 2025 Mar. 21, 2025 Mar. 21, 2025            
Principal/Shares $ 9,104                      
Cost 9,154                      
Market Value $ 9,058                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Quidditch Acquisition, Inc. First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:QuidditchAcquisitionIncMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Hotel, Gaming And Leisure [Member]         Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member]
Spread Above Index (as a percent)   7.00%         7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Interest rate   8.00%         8.00% 8.00% 8.00% 8.00% 8.00% 8.00%
Maturity date   Mar. 21, 2025         Mar. 21, 2025 Mar. 21, 2025 Mar. 21, 2025 Mar. 21, 2025 Mar. 21, 2025 Mar. 21, 2025
Principal/Shares   $ 18,636                    
Cost   18,626                    
Market Value   $ 18,392                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   false         false false false false false false
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: REP Coinvest III- A Omni, LP Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:REPCoinvestIII-AOmniLPMember bcsf:REPCoinvestIII-AOmniLPMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Equity Securities [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member] Transportation Cargo [Member]
Principal/Shares $ 1,377 $ 1,377                    
Cost 1,377 1,377                    
Market Value $ 3,568 $ 2,616                    
Acquisition date Feb. 05, 2021 Feb. 05, 2021                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Reconomy First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Reconomy [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Environmental Industries Sector [Member]   Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member]            
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 8.44%   8.44% 8.44% 8.44% 8.44%            
Maturity date Jun. 24, 2029   Jun. 24, 2029 Jun. 24, 2029 Jun. 24, 2029 Jun. 24, 2029            
Principal/Shares | £         £ 6,118              
Cost $ 7,431                      
Market Value $ 6,834                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember   bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Reconomy First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Reconomy [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Environmental Industries Sector [Member]   Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member]            
Spread Above Index (as a percent) 6.00%   6.00% 6.00% 6.00% 6.00%            
Interest rate 7.19%   7.19% 7.19% 7.19% 7.19%            
Maturity date Jun. 24, 2029   Jun. 24, 2029 Jun. 24, 2029 Jun. 24, 2029 Jun. 24, 2029            
Principal/Shares $ 2,467                      
Cost 2,578                      
Market Value $ 2,418                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Reconomy First Lien Senior Secured Loan 3                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Reconomy [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Environmental Industries Sector [Member]   Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member]            
Maturity date Jun. 24, 2029   Jun. 24, 2029 Jun. 24, 2029 Jun. 24, 2029 Jun. 24, 2029            
Cost $ (78)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Reconomy First Lien Senior Secured Loan 4                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Reconomy [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Environmental Industries Sector [Member]   Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member]            
Maturity date Jun. 24, 2029   Jun. 24, 2029 Jun. 24, 2029 Jun. 24, 2029 Jun. 24, 2029            
Cost $ (78)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Refine Intermediate, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Refine Intermediate, Inc [Member] Refine Intermediate, Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent) 4.50% 4.50% 4.50% 4.50% 4.50% 4.50% 4.50% 4.50% 4.50% 4.50% 4.50% 4.50%
Interest rate 8.17% 5.50% 8.17% 8.17% 8.17% 8.17% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Maturity date Mar. 03, 2027 Mar. 03, 2027 Mar. 03, 2027 Mar. 03, 2027 Mar. 03, 2027 Mar. 03, 2027 Mar. 03, 2027 Mar. 03, 2027 Mar. 03, 2027 Mar. 03, 2027 Mar. 03, 2027 Mar. 03, 2027
Principal/Shares   $ 21,894     £ 1,094              
Cost $ 1,154 21,467                    
Market Value $ 1,094 $ 21,894                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember London Interbank Offered Rate (LIBOR) [Member] bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Refine Intermediate, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Refine Intermediate, Inc [Member] Refine Intermediate, Inc [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Maturity date Sep. 03, 2026 Sep. 03, 2026 Sep. 03, 2026 Sep. 03, 2026 Sep. 03, 2026 Sep. 03, 2026 Sep. 03, 2026 Sep. 03, 2026 Sep. 03, 2026 Sep. 03, 2026 Sep. 03, 2026 Sep. 03, 2026
Cost $ (81) $ (96)                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember   bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Regan Development Holdings Limited First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ReganDevelopmentHoldingsLimitedMember bcsf:ReganDevelopmentHoldingsLimitedMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Spread Above Index (as a percent) 7.00% 6.50% 7.00% 7.00% 7.00% 7.00% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Interest rate 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date Apr. 18, 2023 Apr. 18, 2022 Apr. 18, 2023 Apr. 18, 2023 Apr. 18, 2023 Apr. 18, 2023 Apr. 18, 2022 Apr. 18, 2022 Apr. 18, 2022 Apr. 18, 2022 Apr. 18, 2022 Apr. 18, 2022
Principal/Shares   $ 2,087   € 2,087                
Cost $ 2,274 2,274                    
Market Value $ 1,979 $ 2,326                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Regan Development Holdings Limited First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ReganDevelopmentHoldingsLimitedMember bcsf:ReganDevelopmentHoldingsLimitedMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] bcsf:FirstLienSeniorSecuredLoan2Member First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Spread Above Index (as a percent) 7.00% 6.50% 7.00% 7.00% 7.00% 7.00% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Interest rate 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date Apr. 18, 2023 Apr. 18, 2022 Apr. 18, 2023 Apr. 18, 2023 Apr. 18, 2023 Apr. 18, 2023 Apr. 18, 2022 Apr. 18, 2022 Apr. 18, 2022 Apr. 18, 2022 Apr. 18, 2022 Apr. 18, 2022
Principal/Shares   $ 677   € 677                
Cost $ 768 768                    
Market Value $ 642 $ 754                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Regan Development Holdings Limited First Lien Senior Secured Loan 3                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ReganDevelopmentHoldingsLimitedMember bcsf:ReganDevelopmentHoldingsLimitedMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Spread Above Index (as a percent) 7.00% 6.50% 7.00% 7.00% 7.00% 7.00% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Interest rate 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date Apr. 18, 2023 Apr. 18, 2022 Apr. 18, 2023 Apr. 18, 2023 Apr. 18, 2023 Apr. 18, 2023 Apr. 18, 2022 Apr. 18, 2022 Apr. 18, 2022 Apr. 18, 2022 Apr. 18, 2022 Apr. 18, 2022
Principal/Shares   $ 6,335   € 6,335                
Cost $ 6,868 6,895                    
Market Value $ 5,977 $ 7,041                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Revalize, Inc. First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Revalize, Inc [Member] Revalize, Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 9.42%   9.42% 9.42% 9.42% 9.42%            
Maturity date Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027
Principal/Shares $ 5,371                      
Cost 5,325                      
Cost   $ (133)                    
Market Value $ 5,331 $ (134)                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Revalize, Inc. First Lien Senior Secured Loan - Delayed Draw 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Revalize, Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Maturity date Apr. 15, 2027   Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027            
Cost $ (112)                      
Market Value $ (98)                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Revalize, Inc. First Lien Senior Secured Loan - Delayed Draw Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   Revalize, Inc [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   High Tech Industries [Member]         High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent)   5.25%         5.25% 5.25% 5.25% 5.25% 5.25% 5.25%
Interest rate   6.25%         6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date   Apr. 15, 2027         Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027
Principal/Shares   $ 5,130                    
Cost   5,079                    
Market Value   $ 5,079                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Revalize, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Revalize, Inc [Member] Revalize, Inc [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 9.42%   9.42% 9.42% 9.42% 9.42%            
Maturity date Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027 Apr. 15, 2027
Principal/Shares $ 1,340                      
Cost 1,329                      
Cost   $ (13)                    
Market Value $ 1,330 $ (13)                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: RoC Opco LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:RoCOpcoLLCMember bcsf:RoCOpcoLLCMember                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]
Spread Above Index (as a percent) 8.50%   8.50% 8.50% 8.50% 8.50%            
Interest rate 11.49%   11.49% 11.49% 11.49% 11.49%            
Maturity date Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025
Principal/Shares $ 7,510                      
Cost 7,423                      
Cost   $ (111)                    
Market Value $ 7,485                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: RoC Opco LLCFirst Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:RoCOpcoLLCMember bcsf:RoCOpcoLLCMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]
Spread Above Index (as a percent) 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50%
Interest rate 12.17% 9.50% 12.17% 12.17% 12.17% 12.17% 9.50% 9.50% 9.50% 9.50% 9.50% 9.50%
Maturity date Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025 Feb. 25, 2025
Principal/Shares $ 15,080 $ 40,079                    
Cost 14,907 39,486                    
Market Value $ 15,042 $ 40,079                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Robinson Helicopter Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:RobinsonHelicopterMember                      
Investment, Type [Extensible Enumeration] Common Stock [Member]   Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member]   Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]            
Principal/Shares $ 1,592                      
Cost 1,592                      
Market Value $ 1,592                      
Acquisition date Jun. 30, 2022                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: Robinson Helicopter First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:RobinsonHelicopterMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member]   Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]            
Spread Above Index (as a percent) 6.50%   6.50% 6.50% 6.50% 6.50%            
Interest rate 9.63%   9.63% 9.63% 9.63% 9.63%            
Maturity date Jun. 30, 2028   Jun. 30, 2028 Jun. 30, 2028 Jun. 30, 2028 Jun. 30, 2028            
Principal/Shares $ 36,937                      
Cost 36,129                      
Market Value $ 36,199                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: SAM First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:SAMMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member]   Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member]            
Spread Above Index (as a percent) 11.25%   11.25% 11.25% 11.25% 11.25%            
Interest rate 11.25%   11.25% 11.25% 11.25% 11.25%            
Maturity date May 09, 2028   May 09, 2028 May 09, 2028 May 09, 2028 May 09, 2028            
Principal/Shares $ 32,500                      
Cost 32,213                      
Market Value $ 32,175                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Saltoun First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Saltoun [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Hotel, Gaming And Leisure [Member]   Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member]            
Spread Above Index (as a percent) 10.50%   10.50% 10.50% 10.50% 10.50%            
Interest rate 10.50%   10.50% 10.50% 10.50% 10.50%            
Maturity date Apr. 11, 2028   Apr. 11, 2028 Apr. 11, 2028 Apr. 11, 2028 Apr. 11, 2028            
Principal/Shares $ 4,726                      
Cost 4,726                      
Market Value $ 4,584                      
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Saltoun First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Saltoun [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Hotel, Gaming And Leisure [Member]   Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member] Hotel, Gaming And Leisure [Member]            
Spread Above Index (as a percent) 10.50%   10.50% 10.50% 10.50% 10.50%            
Interest rate 10.50%   10.50% 10.50% 10.50% 10.50%            
Maturity date Apr. 11, 2028   Apr. 11, 2028 Apr. 11, 2028 Apr. 11, 2028 Apr. 11, 2028            
Principal/Shares $ 1,352                      
Cost 1,352                      
Market Value $ 881                      
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Saturn Purchaser Corp. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Saturn Purchaser Corp [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member]   Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]            
Spread Above Index (as a percent) 5.60%   5.60% 5.60% 5.60% 5.60%            
Interest rate 8.54%   8.54% 8.54% 8.54% 8.54%            
Maturity date Jul. 22, 2029   Jul. 22, 2029 Jul. 22, 2029 Jul. 22, 2029 Jul. 22, 2029            
Principal/Shares $ 56,867                      
Cost 56,303                      
Market Value $ 56,299                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Saturn Purchaser Corp. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Saturn Purchaser Corp [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member]   Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]            
Spread Above Index (as a percent) 5.60%   5.60% 5.60% 5.60% 5.60%            
Interest rate 8.62%   8.62% 8.62% 8.62% 8.62%            
Maturity date Jul. 22, 2029   Jul. 22, 2029 Jul. 22, 2029 Jul. 22, 2029 Jul. 22, 2029            
Principal/Shares $ 651                      
Cost 604                      
Market Value $ 602                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Service Master Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ServiceMasterMember bcsf:ServiceMasterMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Principal/Shares $ 350                      
Cost 350 $ 327                    
Market Value $ 397 $ 353                    
Acquisition date Aug. 16, 2021 Aug. 16, 2021                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Service Master First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ServiceMasterMember bcsf:ServiceMasterMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Spread Above Index (as a percent) 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Interest rate 10.42% 8.50% 10.42% 10.42% 10.42% 10.42% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50%
Maturity date Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027
Principal/Shares $ 929 $ 939                    
Cost 914 921                    
Market Value $ 929 $ 939                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Service Master First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ServiceMasterMember bcsf:ServiceMasterMember                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Spread Above Index (as a percent) 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Interest rate 11.10% 8.50% 11.10% 11.10% 11.10% 11.10% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50%
Maturity date Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027
Principal/Shares $ 2,430 $ 1,260                    
Cost 2,357 1,176                    
Market Value $ 2,430 $ 1,260                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Service Master First Lien Senior Secured Loan Two                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:ServiceMasterMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member]   Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member]            
Spread Above Index (as a percent) 8.50%   8.50% 8.50% 8.50% 8.50%            
Interest rate 11.53%   11.53% 11.53% 11.53% 11.53%            
Maturity date Aug. 16, 2027   Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027 Aug. 16, 2027            
Principal/Shares $ 22,000                      
Cost 22,000                      
Market Value $ 22,000                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Smartronix First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:SmartronixMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 6.00%   6.00% 6.00% 6.00% 6.00%            
Interest rate 10.17%   10.17% 10.17% 10.17% 10.17%            
Maturity date Nov. 23, 2027   Nov. 23, 2027 Nov. 23, 2027 Nov. 23, 2027 Nov. 23, 2027            
Principal/Shares $ 12,668                      
Cost 12,441                      
Market Value $ 12,351                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Smartronix First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:SmartronixMember                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Maturity date Nov. 23, 2027   Nov. 23, 2027 Nov. 23, 2027 Nov. 23, 2027 Nov. 23, 2027            
Cost $ (111)                      
Market Value $ (158)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Smartronix RC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:SmartronixRCMember                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Maturity date   Nov. 23, 2028         Nov. 23, 2028 Nov. 23, 2028 Nov. 23, 2028 Nov. 23, 2028 Nov. 23, 2028 Nov. 23, 2028
Cost   $ (124)                    
Market Value   $ (126)                    
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Smartronix T/L First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:SmartronixTLMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate   7.00%         7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date   Nov. 23, 2028         Nov. 23, 2028 Nov. 23, 2028 Nov. 23, 2028 Nov. 23, 2028 Nov. 23, 2028 Nov. 23, 2028
Principal/Shares   $ 36,991                    
Cost   36,260                    
Market Value   $ 36,251                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Solaray, LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Solaray L L C [Member] Solaray L L C [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]
Spread Above Index (as a percent) 5.75% 5.50% 5.75% 5.75% 5.75% 5.75% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 9.40% 6.50% 9.40% 9.40% 9.40% 9.40% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Sep. 11, 2023 Sep. 11, 2023 Sep. 11, 2023 Sep. 11, 2023 Sep. 11, 2023 Sep. 11, 2023 Sep. 11, 2023 Sep. 11, 2023 Sep. 11, 2023 Sep. 11, 2023 Sep. 11, 2023 Sep. 11, 2023
Principal/Shares $ 30,762 $ 41,729                    
Cost 30,762 41,729                    
Market Value $ 30,608 $ 41,729                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Solaray, LLC First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Solaray L L C [Member] Solaray L L C [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]
Spread Above Index (as a percent) 5.75% 5.50% 5.75% 5.75% 5.75% 5.75% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 9.40% 6.50% 9.40% 9.40% 9.40% 9.40% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2023
Principal/Shares $ 14,202 $ 14,276                    
Cost 14,202 14,276                    
Market Value $ 14,131 $ 14,276                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Solaray, LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Solaray L L C [Member] Solaray L L C [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]
Spread Above Index (as a percent) 3.55% 4.50% 3.55% 3.55% 3.55% 3.55% 4.50% 4.50% 4.50% 4.50% 4.50% 4.50%
Interest rate 8.16% 5.50% 8.16% 8.16% 8.16% 8.16% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Maturity date Sep. 09, 2023 Sep. 09, 2022 Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2023 Sep. 09, 2022 Sep. 09, 2022 Sep. 09, 2022 Sep. 09, 2022 Sep. 09, 2022 Sep. 09, 2022
Principal/Shares $ 5,950 $ 907                    
Cost 5,950 895                    
Market Value $ 5,950 $ 907                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Spring Finco BV, First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Spring Finco BV [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 6.00%   6.00% 6.00% 6.00% 6.00%            
Interest rate 8.07%   8.07% 8.07% 8.07% 8.07%            
Maturity date Jul. 15, 2029   Jul. 15, 2029 Jul. 15, 2029 Jul. 15, 2029 Jul. 15, 2029            
Principal/Shares $ 49,343                      
Cost 5,160                      
Market Value $ 4,646                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Spring Finco BV, First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Spring Finco BV [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Maturity date Jul. 15, 2029   Jul. 15, 2029 Jul. 15, 2029 Jul. 15, 2029 Jul. 15, 2029            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Stanton Carpet Second Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:StantonCarpetMember bcsf:StantonCarpetMember                    
Investment, Type [Extensible Enumeration] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member]
Spread Above Index (as a percent) 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00%
Interest rate 12.75% 10.00% 12.75% 12.75% 12.75% 12.75% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00%
Principal/Shares $ 11,434 $ 19,664                    
Cost 11,229 19,277                    
Market Value $ 11,434 $ 19,271                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: SumUp Holdings Luxembourg S.a.r.l. First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:SumUpHoldingsLuxembourgSarlMember bcsf:SumUpHoldingsLuxembourgSarlMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent) 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50%
Interest rate 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00%
Maturity date Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026
Principal/Shares | €       € 6,650       € 6,650        
Cost $ 7,945             7,939        
Market Value $ 6,519             € 7,561        
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember
Investment, Interest Rate, Floor 1.50%   1.50% 1.50% 1.50% 1.50%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: SumUp Holdings Luxembourg S.a.r.l. First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:SumUpHoldingsLuxembourgSarlMember bcsf:SumUpHoldingsLuxembourgSarlMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent) 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50%
Interest rate 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00% 10.00%
Maturity date Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026 Feb. 17, 2026
Principal/Shares       € 155         £ 10,055      
Cost $ 180               11,700      
Market Value $ 152               £ 11,432      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember London Interbank Offered Rate (LIBOR) [Member] bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.50%   1.50% 1.50% 1.50% 1.50%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: SunMed Group Holdings, LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:SunMedGroupHoldingsLLCMember bcsf:SunMedGroupHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember First Lien Senior Secured Loan [Member] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Spread Above Index (as a percent) 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate 8.82% 6.50% 8.82% 8.82% 8.82% 8.82% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Jun. 16, 2027 Jun. 16, 2028 Jun. 16, 2027 Jun. 16, 2027 Jun. 16, 2027 Jun. 16, 2027 Jun. 16, 2028 Jun. 16, 2028 Jun. 16, 2028 Jun. 16, 2028 Jun. 16, 2028 Jun. 16, 2028
Principal/Shares $ 590 $ 18,510                    
Cost 573 18,204                    
Market Value $ 550 $ 18,510                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: SunMed Group Holdings, LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:SunMedGroupHoldingsLLCMember bcsf:SunMedGroupHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] bcsf:FirstLienSeniorSecuredLoanRevolverMember First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Spread Above Index (as a percent) 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate 9.42% 6.50% 9.42% 9.42% 9.42% 9.42% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Jun. 16, 2028 Jun. 16, 2027 Jun. 16, 2028 Jun. 16, 2028 Jun. 16, 2028 Jun. 16, 2028 Jun. 16, 2027 Jun. 16, 2027 Jun. 16, 2027 Jun. 16, 2027 Jun. 16, 2027 Jun. 16, 2027
Principal/Shares $ 8,716 $ 197                    
Cost 8,585 177                    
Market Value $ 8,433 $ 197                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Superna Inc. Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Superna Inc [Member]                      
Investment, Type [Extensible Enumeration] Common Stock [Member]   Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Principal/Shares $ 1,463                      
Cost 1,463                      
Market Value $ 1,514                      
Acquisition date Mar. 08, 2022                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: Superna Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Superna Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Spread Above Index (as a percent) 6.50%   6.50% 6.50% 6.50% 6.50%            
Interest rate 9.52%   9.52% 9.52% 9.52% 9.52%            
Maturity date Mar. 06, 2028   Mar. 06, 2028 Mar. 06, 2028 Mar. 06, 2028 Mar. 06, 2028            
Principal/Shares $ 14,958                      
Cost 14,675                      
Market Value $ 14,808                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Superna Inc. First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Superna Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Maturity date Mar. 06, 2028   Mar. 06, 2028 Mar. 06, 2028 Mar. 06, 2028 Mar. 06, 2028            
Cost $ (24)                      
Market Value $ (26)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Superna Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Superna Inc [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Maturity date Mar. 06, 2028   Mar. 06, 2028 Mar. 06, 2028 Mar. 06, 2028 Mar. 06, 2028            
Cost $ (24)                      
Market Value $ (26)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Surrey Bidco Limited First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:SurreyBidcoLimitedMember bcsf:SurreyBidcoLimitedMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member]
Spread Above Index (as a percent) 6.00% 7.00% 6.00% 6.00% 6.00% 6.00% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Interest rate 7.97% 7.50% 7.97% 7.97% 7.97% 7.97% 7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Maturity date May 11, 2026 May 11, 2026 May 11, 2026 May 11, 2026 May 11, 2026 May 11, 2026 May 11, 2026 May 11, 2026 May 11, 2026 May 11, 2026 May 11, 2026 May 11, 2026
Principal/Shares | £         £ 54       £ 50      
Cost $ 67               62      
Market Value $ 52               £ 60      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:SterlingOvernightInterbankAverageRateMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:SterlingOvernightInterbankAverageRateMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember bcsf:BritishPoundSterlingLondonInterbankOfferedRateLiborMember
Investment, Interest Rate, Floor 0.50%   0.50% 0.50% 0.50% 0.50%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Swoogo LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Swoogo L L C [Member] Swoogo L L C [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Maturity date Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026
Cost $ (21) $ (25)                    
Market Value $ (22) $ (25)                    
Investment, Interest Rate, Floor 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Swoogo LLC First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Swoogo L L C [Member] Swoogo L L C [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 8.00% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00% 8.00%
Interest rate 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00% 9.00%
Maturity date Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026 Dec. 09, 2026
Principal/Shares $ 2,330 $ 2,330                    
Cost 2,290 2,284                    
Market Value $ 2,289 $ 2,283                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: TA/Weg Holdings First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:TAWegHoldingsMember bcsf:TAWegHoldingsMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member]
Spread Above Index (as a percent) 6.00% 5.75% 6.00% 6.00% 6.00% 6.00% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate 7.67% 6.75% 7.67% 7.67% 7.67% 7.67% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date Oct. 02, 2025 Oct. 02, 2027 Oct. 02, 2025 Oct. 02, 2025 Oct. 02, 2025 Oct. 02, 2025 Oct. 02, 2027 Oct. 02, 2027 Oct. 02, 2027 Oct. 02, 2027 Oct. 02, 2027 Oct. 02, 2027
Principal/Shares $ 2,379 $ 9,495                    
Cost 2,369 9,495                    
Market Value $ 2,379 $ 9,495                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: TA/Weg Holdings First Lien Senior Secured Loan - Delayed Draw 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:TAWegHoldingsMember bcsf:TAWegHoldingsMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member] Fire Finance [Member]
Spread Above Index (as a percent) 6.00% 5.75% 6.00% 6.00% 6.00% 6.00% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate 9.41% 6.75% 9.41% 9.41% 9.41% 9.41% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date Oct. 02, 2025 Oct. 02, 2027 Oct. 02, 2025 Oct. 02, 2025 Oct. 02, 2025 Oct. 02, 2025 Oct. 02, 2027 Oct. 02, 2027 Oct. 02, 2027 Oct. 02, 2027 Oct. 02, 2027 Oct. 02, 2027
Principal/Shares $ 9,423 $ 2,392                    
Cost 9,423 2,381                    
Market Value $ 9,423 $ 2,392                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: TCFIII Owl Finance, LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:TCFIIIOwlFinanceLLCMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Capital Equipment Sector [Member]   Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member]            
Spread Above Index (as a percent) 12.00%   12.00% 12.00% 12.00% 12.00%            
Interest rate 12.00%   12.00% 12.00% 12.00% 12.00%            
Maturity date Jan. 30, 2027   Jan. 30, 2027 Jan. 30, 2027 Jan. 30, 2027 Jan. 30, 2027            
Principal/Shares $ 4,697                      
Cost 4,636                      
Market Value $ 4,603                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: TEI Holdings Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] T E I Holdings Inc [Member] T E I Holdings Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent) 5.75% 7.00% 5.75% 5.75% 5.75% 5.75% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
PIK   (1.25%)         (1.25%) (1.25%) (1.25%) (1.25%) (1.25%) (1.25%)
Interest rate 8.87% 8.25% 8.87% 8.87% 8.87% 8.87% 8.25% 8.25% 8.25% 8.25% 8.25% 8.25%
Maturity date Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026 Dec. 23, 2026
Principal/Shares $ 38,475 $ 48,720                    
Cost 38,283 48,350                    
Market Value $ 38,475 $ 48,720                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: TEI Holdings Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] T E I Holdings Inc [Member] T E I Holdings Inc [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   6.00%         6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate   7.00%         7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date Dec. 23, 2025 Dec. 23, 2025 Dec. 23, 2025 Dec. 23, 2025 Dec. 23, 2025 Dec. 23, 2025 Dec. 23, 2025 Dec. 23, 2025 Dec. 23, 2025 Dec. 23, 2025 Dec. 23, 2025 Dec. 23, 2025
Principal/Shares   $ 458                    
Cost   412                    
Cost $ (40)                      
Market Value $ 5 $ 458                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: TGI Sport Bidco Pty Ltd First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   T G I Sport Bidco Pty Ltd [Member]                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Media: Advertising, Printing & Publishing         Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing
Maturity date   Apr. 30, 2027         Apr. 30, 2027 Apr. 30, 2027 Apr. 30, 2027 Apr. 30, 2027 Apr. 30, 2027 Apr. 30, 2027
Market Value             $ (151)          
Investment, Interest Rate, Floor   0.50%         0.50% 0.50% 0.50% 0.50% 0.50% 0.50%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: TGI Sport Bidco Pty Ltd First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] T G I Sport Bidco Pty Ltd [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Media: Advertising, Printing & Publishing   Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing            
Spread Above Index (as a percent) 7.00%   7.00% 7.00% 7.00% 7.00%            
Interest rate 10.60%   10.60% 10.60% 10.60% 10.60%            
Maturity date Apr. 30, 2026   Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026            
Principal/Shares     $ 4,166                  
Cost $ 2,720                      
Market Value $ 2,520                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: TGI Sport Bidco Pty Ltd First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] T G I Sport Bidco Pty Ltd [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Media: Advertising, Printing & Publishing   Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing            
Spread Above Index (as a percent) 7.00%   7.00% 7.00% 7.00% 7.00%            
Interest rate 8.50%   8.50% 8.50% 8.50% 8.50%            
Maturity date Apr. 30, 2026   Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026            
Principal/Shares     $ 97                  
Cost $ 75                      
Market Value $ 59                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:BankBillSwapBidRateMember   bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember            
Investment, Interest Rate, Floor 0.50%   0.50% 0.50% 0.50% 0.50%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: TGI Sport Bidco Pty Ltd First Lien Senior Secured Loan Bbsw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   T G I Sport Bidco Pty Ltd [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Media: Advertising, Printing & Publishing         Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing Media: Advertising, Printing & Publishing
Spread Above Index (as a percent)   7.00%         7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Interest rate   7.50%         7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Maturity date   Apr. 30, 2026         Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026
Principal/Shares             $ 97          
Cost             75          
Market Value             $ 67          
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:BankBillSwapBidRateMember         bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember
Investment, Interest Rate, Floor   0.50%         0.50% 0.50% 0.50% 0.50% 0.50% 0.50%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: TLC Holdco LP Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:TLCHoldcoLPMember bcsf:TLCHoldcoLPMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member]
Principal/Shares $ 1,281 $ 1,188                    
Cost $ 1,221 1,186                    
Market Value   $ 431                    
Acquisition date Oct. 11, 2019 Oct. 11, 2019                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: TLC Purchaser, Inc. First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] T L C Purchaser, Inc [Member] T L C Purchaser, Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member]
Maturity date   Oct. 13, 2025         Oct. 13, 2025 Oct. 13, 2025 Oct. 13, 2025 Oct. 13, 2025 Oct. 13, 2025 Oct. 13, 2025
Cost $ (90) $ (45)                    
Market Value $ (1,409) $ (854)                    
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: TLC Purchaser, Inc.First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] T L C Purchaser, Inc [Member] T L C Purchaser, Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member]
Spread Above Index (as a percent) 6.25% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Interest rate 11.54% 7.25% 11.54% 11.54% 11.54% 11.54% 7.25% 7.25% 7.25% 7.25% 7.25% 7.25%
Maturity date   Oct. 13, 2025         Oct. 13, 2025 Oct. 13, 2025 Oct. 13, 2025 Oct. 13, 2025 Oct. 13, 2025 Oct. 13, 2025
Principal/Shares $ 35,240 $ 41,066                    
Cost 34,579 40,511                    
Market Value $ 28,720 $ 36,137                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: TLC Purchaser, Inc.First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] T L C Purchaser, Inc [Member] T L C Purchaser, Inc [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member]
Spread Above Index (as a percent) 6.25% 5.25% 6.25% 6.25% 6.25% 6.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25%
Interest rate 9.37% 8.50% 9.37% 9.37% 9.37% 9.37% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50%
Maturity date   Oct. 13, 2025         Oct. 13, 2025 Oct. 13, 2025 Oct. 13, 2025 Oct. 13, 2025 Oct. 13, 2025 Oct. 13, 2025
Principal/Shares $ 7,693 $ 6,408                    
Cost 7,536 6,296                    
Market Value $ 5,932 $ 5,340                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Tangent Technologies Acquisition, LLCSecond Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:TangentTechnologiesAcquisitionLLCMember                      
Investment, Type [Extensible Enumeration] Second Lien Senior Secured Loan [Member]   Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Durable Sector [Member]   Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member] Consumer Goods Durable Sector [Member]            
Spread Above Index (as a percent) 8.75%   8.75% 8.75% 8.75% 8.75%            
Interest rate 9.99%   9.99% 9.99% 9.99% 9.99%            
Principal/Shares $ 8,915                      
Cost 8,752                      
Market Value $ 8,915                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: TecoStar Holdings, Inc. Second Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:TecoStarHoldingsIncMember bcsf:TecoStarHoldingsIncMember                    
Investment, Type [Extensible Enumeration] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member] Healthcare And Pharmaceuticals [Member]
Spread Above Index (as a percent) 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50% 8.50%
Interest rate 11.31% 9.50% 11.31% 11.31% 11.31% 11.31% 9.50% 9.50% 9.50% 9.50% 9.50% 9.50%
Maturity date Nov. 01, 2024 Nov. 01, 2024 Nov. 01, 2024 Nov. 01, 2024 Nov. 01, 2024 Nov. 01, 2024 Nov. 01, 2024 Nov. 01, 2024 Nov. 01, 2024 Nov. 01, 2024 Nov. 01, 2024 Nov. 01, 2024
Principal/Shares $ 9,472 $ 9,472                    
Cost 9,381 9,354                    
Market Value $ 9,022 $ 8,951                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Thrasio, LLC First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] New Look Vision Group [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Retail Sector [Member]   Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]            
Spread Above Index (as a percent) 7.00%   7.00% 7.00% 7.00% 7.00%            
Interest rate 11.17%   11.17% 11.17% 11.17% 11.17%            
Maturity date Dec. 18, 2026   Dec. 18, 2026 Dec. 18, 2026 Dec. 18, 2026 Dec. 18, 2026            
Principal/Shares $ 8,506                      
Cost 8,320                      
Market Value $ 7,826                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Thrasio, LLC First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:ThrasioLLCMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Retail Sector [Member]         Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]
Spread Above Index (as a percent)   7.00%         7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Interest rate   8.00%         8.00% 8.00% 8.00% 8.00% 8.00% 8.00%
Maturity date   Dec. 18, 2026         Dec. 18, 2026 Dec. 18, 2026 Dec. 18, 2026 Dec. 18, 2026 Dec. 18, 2026 Dec. 18, 2026
Principal/Shares   $ 21,746                    
Cost   21,241                    
Market Value   $ 21,746                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Tidel Engineering, L.P. First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:TidelEngineeringLPMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Capital Equipment Sector [Member]         Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date   Mar. 01, 2024         Mar. 01, 2024 Mar. 01, 2024 Mar. 01, 2024 Mar. 01, 2024 Mar. 01, 2024 Mar. 01, 2024
Principal/Shares   $ 38,155                    
Cost   38,155                    
Market Value   $ 38,155                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Tidel Engineering, L.P. First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:TidelEngineeringLPMember                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoan2Member         bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member bcsf:FirstLienSeniorSecuredLoan2Member
Investment, Industry Sector [Extensible Enumeration]   Capital Equipment Sector [Member]         Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member] Capital Equipment Sector [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date   Mar. 01, 2024         Mar. 01, 2024 Mar. 01, 2024 Mar. 01, 2024 Mar. 01, 2024 Mar. 01, 2024 Mar. 01, 2024
Principal/Shares   $ 6,337                    
Cost   6,274                    
Market Value   $ 6,336                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Titan Cloud Software, Inc First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Titan Cloud Software, Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Environmental Industries Sector [Member]   Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member]            
Spread Above Index (as a percent) 6.60%   6.60% 6.60% 6.60% 6.60%            
Interest rate 9.63%   9.63% 9.63% 9.63% 9.63%            
Maturity date Sep. 07, 2029   Sep. 07, 2029 Sep. 07, 2029 Sep. 07, 2029 Sep. 07, 2029            
Principal/Shares | €       € 25,714                
Cost $ 25,457                      
Market Value $ 25,457                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Titan Cloud Software, Inc First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Titan Cloud Software, Inc [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Environmental Industries Sector [Member]   Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member]            
Maturity date Sep. 07, 2029   Sep. 07, 2029 Sep. 07, 2029 Sep. 07, 2029 Sep. 07, 2029            
Cost $ (113)                      
Market Value $ (114)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Titan Cloud Software, Inc First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Titan Cloud Software, Inc [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Environmental Industries Sector [Member]   Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member] Environmental Industries Sector [Member]            
Maturity date Sep. 07, 2028   Sep. 07, 2028 Sep. 07, 2028 Sep. 07, 2028 Sep. 07, 2028            
Cost $ (57)                      
Market Value $ (57)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Toro Private Investments II, L.P. Equity Interest 2                        
Consolidated Schedule of Investments                        
Acquisition date   Apr. 02, 2019                    
Investment, Identifier [Axis]: Toro Private Investments II, LP Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ToroPrivateInvestmentsIILPMember bcsf:ToroPrivateInvestmentsIILPMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Equity Securities [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member] Equity Securities [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Consumer [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer [Member] Transportation Consumer [Member] Transportation Consumer [Member] Transportation Consumer [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member]
Principal/Shares $ 3,090 $ 3,090                    
Cost 3,090 3,090                    
Market Value $ 1,173 $ 1,353                    
Acquisition date Apr. 02, 2019                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Toro Private Investments II, LP First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ToroPrivateInvestmentsIILPMember bcsf:ToroPrivateInvestmentsIILPMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Consumer [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer [Member] Transportation Consumer [Member] Transportation Consumer [Member] Transportation Consumer [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member]
Spread Above Index (as a percent) 5.00% 6.75% 5.00% 5.00% 5.00% 5.00% 6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
PIK (1.75%)   (1.75%) (1.75%) (1.75%) (1.75%)            
Interest rate 10.42% 6.90% 10.42% 10.42% 10.42% 10.42% 6.90% 6.90% 6.90% 6.90% 6.90% 6.90%
Maturity date May 29, 2026 May 29, 2026 May 29, 2026 May 29, 2026 May 29, 2026 May 29, 2026 May 29, 2026 May 29, 2026 May 29, 2026 May 29, 2026 May 29, 2026 May 29, 2026
Principal/Shares $ 6,744 $ 6,706                    
Cost 5,182 4,846                    
Market Value $ 4,794 $ 5,603                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Toro Private Investments ll, LP First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ToroPrivateInvestmentsIILPMember bcsf:ToroPrivateInvestmentsIILPMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Transportation Consumer [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer [Member] Transportation Consumer [Member] Transportation Consumer [Member] Transportation Consumer [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member] Transportation Consumer Non Controlled Non Affiliate [Member]
Spread Above Index (as a percent) 1.50% 1.50% 1.50% 1.50% 1.50% 1.50% 1.50% 1.50% 1.50% 1.50% 1.50% 1.50%
PIK (7.25%) (7.25%) (7.25%) (7.25%) (7.25%) (7.25%) (7.25%) (7.25%) (7.25%) (7.25%) (7.25%) (7.25%)
Interest rate 12.42% 9.75% 12.42% 12.42% 12.42% 12.42% 9.75% 9.75% 9.75% 9.75% 9.75% 9.75%
Maturity date Feb. 28, 2025 Feb. 28, 2025 Feb. 28, 2025 Feb. 28, 2025 Feb. 28, 2025 Feb. 28, 2025 Feb. 28, 2025 Feb. 28, 2025 Feb. 28, 2025 Feb. 28, 2025 Feb. 28, 2025 Feb. 28, 2025
Principal/Shares $ 394 $ 366                    
Cost 391 363                    
Market Value $ 391 $ 377                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true false true true true true false false false false false false
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Utimaco, Inc. Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:UtimacoIncMember                      
Investment, Type [Extensible Enumeration] Common Stock [Member]   Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Principal/Shares $ 1                      
Cost 2,123                      
Market Value $ 2,006                      
Acquisition date Jun. 28, 2022                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: Utimaco, Inc. First Lien Senior Secured Loan Euibor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:UtimacoIncMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Spread Above Index (as a percent) 6.00%   6.00% 6.00% 6.00% 6.00%            
Interest rate 6.58%   6.58% 6.58% 6.58% 6.58%            
Maturity date May 13, 2029   May 13, 2029 May 13, 2029 May 13, 2029 May 13, 2029            
Principal/Shares $ 92                      
Cost 100                      
Market Value $ 91                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Utimaco, Inc. First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:UtimacoIncMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   High Tech Industries [Member]         High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent)   4.00%         4.00% 4.00% 4.00% 4.00% 4.00% 4.00%
Interest rate   4.10%         4.10% 4.10% 4.10% 4.10% 4.10% 4.10%
Maturity date   Aug. 09, 2027         Aug. 09, 2027 Aug. 09, 2027 Aug. 09, 2027 Aug. 09, 2027 Aug. 09, 2027 Aug. 09, 2027
Principal/Shares   $ 148                    
Cost   146                    
Market Value   $ 148                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Utimaco, Inc. First Lien Senior Secured Loan Sofr 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:UtimacoIncMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Spread Above Index (as a percent) 6.00%   6.00% 6.00% 6.00% 6.00%            
Interest rate 8.83%   8.83% 8.83% 8.83% 8.83%            
Maturity date May 13, 2029   May 13, 2029 May 13, 2029 May 13, 2029 May 13, 2029            
Principal/Shares $ 128                      
Cost 127                      
Market Value $ 128                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Utimaco, Inc. First Lien Senior Secured Loan Sofr 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:UtimacoIncMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Spread Above Index (as a percent) 6.00%   6.00% 6.00% 6.00% 6.00%            
Interest rate 8.83%   8.83% 8.83% 8.83% 8.83%            
Maturity date May 13, 2029   May 13, 2029 May 13, 2029 May 13, 2029 May 13, 2029            
Principal/Shares $ 262                      
Cost 259                      
Market Value $ 262                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]   Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Utimaco, Inc. Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:UtimacoIncMember                      
Investment, Type [Extensible Enumeration] Preferred Stock [Member]   Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Principal/Shares $ 1                      
Cost 2,123                      
Market Value $ 2,006                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: V Global Holdings LLCFirst Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] V Global Holdings L L C [Member] V Global Holdings L L C [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member]
Spread Above Index (as a percent) 5.75% 6.00% 5.75% 5.75% 5.75% 5.75% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate 8.99% 7.00% 8.99% 8.99% 8.99% 8.99% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027
Principal/Shares $ 5,876 $ 24,813                    
Cost 5,795 24,242                    
Market Value $ 5,772 $ 24,813                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.75% 1.00% 0.75% 0.75% 0.75% 0.75% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: V Global Holdings LLCFirst Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] V Global Holdings L L C [Member] V Global Holdings L L C [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member]
Spread Above Index (as a percent) 5.75% 5.00% 5.75% 5.75% 5.75% 5.75% 5.00% 5.00% 5.00% 5.00% 5.00% 5.00%
Interest rate 8.45% 8.25% 8.45% 8.45% 8.45% 8.45% 8.25% 8.25% 8.25% 8.25% 8.25% 8.25%
Maturity date Dec. 22, 2025 Dec. 22, 2025 Dec. 22, 2025 Dec. 22, 2025 Dec. 22, 2025 Dec. 22, 2025 Dec. 22, 2025 Dec. 22, 2025 Dec. 22, 2025 Dec. 22, 2025 Dec. 22, 2025 Dec. 22, 2025
Principal/Shares $ 1,351 $ 2,050                    
Cost 1,192 1,893                    
Market Value $ 1,182 $ 2,050                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] us-gaap:PrimeRateMember Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] Secured Overnight Financing Rate (SOFR) Overnight Index Swap Rate [Member] us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember
Investment, Interest Rate, Floor 0.75% 1.00% 0.75% 0.75% 0.75% 0.75% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: V Global Holdings Llc First Lien Senior Secured Loan Two                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] V Global Holdings L L C [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Chemicals, Plastics and Rubber [Member]   Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member] Chemicals, Plastics and Rubber [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 6.50%   6.50% 6.50% 6.50% 6.50%            
Maturity date Dec. 22, 2027   Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027 Dec. 22, 2027            
Principal/Shares | €       € 100                
Cost $ 103                      
Market Value $ 97                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 0.75%   0.75% 0.75% 0.75% 0.75%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: VPARK BIDCO AB First Lien Senior Secured Loan Cibor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:VPARKBIDCOABMember bcsf:VPARKBIDCOABMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 4.00% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00%
Interest rate 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
Maturity date Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025
Principal/Shares       € 570             kr 570  
Cost $ 92                   92  
Market Value $ 75                   kr 87  
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:CopenhagenInterbankOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CopenhagenInterbankOfferedRateMember bcsf:CopenhagenInterbankOfferedRateMember bcsf:CopenhagenInterbankOfferedRateMember bcsf:CopenhagenInterbankOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember bcsf:CanadianDollarOfferedRateMember
Investment, Interest Rate, Floor 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: VPARK BIDCO AB First Lien Senior Secured Loan Nibor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:VPARKBIDCOABMember bcsf:VPARKBIDCOABMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 4.00% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00% 4.00%
Interest rate 6.60% 4.75% 6.60% 6.60% 6.60% 6.60% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
Maturity date Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025 Mar. 10, 2025
Principal/Shares $ 740                     kr 740
Cost 93                     93
Market Value $ 68                     kr 84
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:NorwegianInterbankOfferedRateMember bcsf:NorwegianInterbankOfferedRateMember bcsf:NorwegianInterbankOfferedRateMember bcsf:NorwegianInterbankOfferedRateMember bcsf:NorwegianInterbankOfferedRateMember bcsf:NorwegianInterbankOfferedRateMember bcsf:NorwegianInterbankOfferedRateMember bcsf:NorwegianInterbankOfferedRateMember bcsf:NorwegianInterbankOfferedRateMember bcsf:NorwegianInterbankOfferedRateMember bcsf:NorwegianInterbankOfferedRateMember bcsf:NorwegianInterbankOfferedRateMember
Investment, Interest Rate, Floor 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75% 0.75%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Ventiv Holdco, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] Ventiv Holdco, Inc [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member]   High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]            
Maturity date Sep. 03, 2025   Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025            
Cost $ (34)                      
Market Value $ (68)                      
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Ventiv Holdco, Inc. First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Ventiv Holdco, Inc [Member] Ventiv Holdco, Inc [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Spread Above Index (as a percent) 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 9.17% 6.50% 9.17% 9.17% 9.17% 9.17% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025
Principal/Shares $ 13,807 $ 23,812                    
Cost 13,693 23,576                    
Market Value $ 13,531 $ 23,812                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Ventiv Topco, Inc. Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:VentivTopcoIncMember bcsf:VentivTopcoIncMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Principal/Shares $ 28 $ 28                    
Cost 2,833 2,833                    
Market Value $ 2,121 $ 2,755                    
Acquisition date Sep. 03, 2019 Sep. 03, 2019                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: Ventiv Topco, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:VentivTopcoIncMember                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   High Tech Industries [Member]         High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member] High Tech Industries [Member]
Maturity date   Sep. 03, 2025         Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025 Sep. 03, 2025
Cost   $ (38)                    
Investment, Interest Rate, Floor   0.00%         0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: WCI Gigawatt Purchaser DD T/L First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   W C I Gigawatt Purchaser D D T L [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date   Nov. 19, 2027         Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027
Principal/Shares   $ 3,182                    
Cost   3,076                    
Market Value   $ 3,074                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: WCI Gigawatt Purchaser First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:WCIGigawattPurchaserMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 8.73%   8.73% 8.73% 8.73% 8.73%            
Maturity date Nov. 19, 2027   Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027            
Principal/Shares $ 1,447                      
Cost 1,446                      
Market Value $ 1,415                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: WCI Gigawatt Purchaser First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:WCIGigawattPurchaserMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 8.73%   8.73% 8.73% 8.73% 8.73%            
Maturity date Nov. 19, 2027   Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027            
Principal/Shares $ 3,195                      
Cost 3,101                      
Market Value $ 3,086                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: WCI Gigawatt Purchaser First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:WCIGigawattPurchaserMember                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 8.51%   8.51% 8.51% 8.51% 8.51%            
Maturity date Nov. 19, 2027   Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027            
Principal/Shares $ 965                      
Cost 903                      
Market Value $ 893                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: WCI Gigawatt Purchaser R/C First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:WCIGigawattPurchaserRCMember                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Maturity date   Nov. 19, 2027         Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027
Cost   $ (71)                    
Market Value   $ (72)                    
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: WCI Gigawatt Purchaser T/L First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:WCIGigawattPurchaserTLMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date   Nov. 19, 2027         Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027 Nov. 19, 2027
Principal/Shares   $ 22,304                    
Cost   21,809                    
Market Value   $ 21,802                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: WCI-HSG HOLDCO, LLC Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:WCI-HSGHOLDCOLLCMember bcsf:WCI-HSGHOLDCOLLCMember                    
Investment, Type [Extensible Enumeration] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Principal/Shares $ 675 $ 675                    
Cost 675 675                    
Market Value $ 2,217 $ 1,993                    
Acquisition date Feb. 22, 2019 Feb. 22, 2019                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: WCI-HSG Purchaser, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:WCI-HSGPurchaserIncMember bcsf:WCI-HSGPurchaserIncMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 4.50% 4.75% 4.50% 4.50% 4.50% 4.50% 4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
Interest rate 7.62% 5.75% 7.62% 7.62% 7.62% 7.62% 5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Maturity date Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025
Principal/Shares $ 8,644 $ 17,422                    
Cost 8,591 17,285                    
Market Value $ 8,644 $ 17,422                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: WCI-HSG Purchaser, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:WCI-HSGPurchaserIncMember bcsf:WCI-HSGPurchaserIncMember                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent)   4.75%         4.75% 4.75% 4.75% 4.75% 4.75% 4.75%
Interest rate   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Maturity date Feb. 22, 2025 Feb. 24, 2025 Feb. 22, 2025 Feb. 22, 2025 Feb. 22, 2025 Feb. 22, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025 Feb. 24, 2025
Principal/Shares   $ 1,209                    
Cost   1,190                    
Cost $ (4)                      
Market Value   $ 1,209                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: WP CPP Holdings, LLC Senior Second Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:WPCPPHoldingsLLCMember bcsf:WPCPPHoldingsLLCMember                    
Investment, Type [Extensible Enumeration] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member] Second Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 7.75% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75% 7.75%
Interest rate 10.56% 8.75% 10.56% 10.56% 10.56% 10.56% 8.75% 8.75% 8.75% 8.75% 8.75% 8.75%
Maturity date Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026 Apr. 30, 2026
Principal/Shares $ 11,724 $ 11,724                    
Cost 11,658 11,646                    
Market Value $ 10,815 $ 11,495                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true false true true true true false false false false false false
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: WSP Initial Term Loan First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] WSP Initial Term Loan WSP Initial Term Loan                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member]
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 9.37%   9.37% 9.37% 9.37% 9.37%            
Maturity date Apr. 27, 2027 Apr. 27, 2023 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2023 Apr. 27, 2023 Apr. 27, 2023 Apr. 27, 2023 Apr. 27, 2023 Apr. 27, 2023
Principal/Shares $ 6,018                      
Cost 5,916                      
Cost   $ (36)                    
Market Value $ 5,626 $ (31)                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: WSP Initial Term Loan First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] WSP Initial Term Loan                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Wholesale Sector [Member]   Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member]            
Maturity date Apr. 27, 2027   Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027            
Cost $ (15)                      
Market Value $ (117)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: WSP Initial Term Loan First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   WSP Initial Term Loan                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Consumer Goods Wholesale Sector [Member]         Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member]
Spread Above Index (as a percent)   6.25%         6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Interest rate   7.25%         7.25% 7.25% 7.25% 7.25% 7.25% 7.25%
Maturity date   Apr. 27, 2027         Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027
Principal/Shares   $ 12,251                    
Cost   12,017                    
Market Value   $ 12,037                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: WSP LP Interest Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:WSPLPInterestMember bcsf:WSPLPInterestMember                    
Investment, Type [Extensible Enumeration] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member]
Principal/Shares $ 2,898 $ 2,898                    
Cost 2,898 2,898                    
Market Value $ 1,559 $ 2,829                    
Acquisition date Aug. 31, 2021 Aug. 31, 2021                    
Investment, Restriction Status [true false] true true true true true true true true true true true true
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] true true true true true true true true true true true true
Investment, Identifier [Axis]: WSP Revolving Loan First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:WSPLPInterestMember WSP Revolving Loan                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member] Consumer Goods Wholesale Sector [Member]
Spread Above Index (as a percent) 6.25%   6.25% 6.25% 6.25% 6.25%            
Interest rate 6.25%   6.25% 6.25% 6.25% 6.25%            
Maturity date Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027 Apr. 27, 2027
Principal/Shares $ 47                      
Cost 40                      
Cost   $ (9)                    
Market Value $ 18 $ (8)                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00% 1.00% 0.00% 0.00% 0.00% 0.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: WU Holdco, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] WU Holdco, Inc. WU Holdco, Inc.                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]
Spread Above Index (as a percent) 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate 9.17% 6.50% 9.17% 9.17% 9.17% 9.17% 6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026
Principal/Shares $ 37,775 $ 44,452                    
Cost 37,341 43,847                    
Market Value $ 35,886 $ 44,452                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: WU Holdco, Inc. First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] WU Holdco, Inc.                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Non Durable Sector [Member]   Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]            
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 9.17%   9.17% 9.17% 9.17% 9.17%            
Maturity date Mar. 26, 2026   Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026            
Principal/Shares $ 1,704                      
Cost 1,677                      
Market Value $ 1,619                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: WU Holdco, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   WU Holdco, Inc.                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Consumer Goods Non Durable Sector [Member]         Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]
Spread Above Index (as a percent)   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate   5.72%         5.72% 5.72% 5.72% 5.72% 5.72% 5.72%
Maturity date   Mar. 26, 2025         Mar. 26, 2025 Mar. 26, 2025 Mar. 26, 2025 Mar. 26, 2025 Mar. 26, 2025 Mar. 26, 2025
Principal/Shares   $ 1,690                    
Cost   1,656                    
Market Value   $ 1,690                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   0.00%         0.00% 0.00% 0.00% 0.00% 0.00% 0.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: WU Holdco, Inc. First Lien Senior Secured Loan - Revolver Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] WU Holdco, Inc.                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Consumer Goods Non Durable Sector [Member]   Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]            
Spread Above Index (as a percent) 5.50%   5.50% 5.50% 5.50% 5.50%            
Interest rate 9.14%   9.14% 9.14% 9.14% 9.14%            
Maturity date Mar. 26, 2025   Mar. 26, 2025 Mar. 26, 2025 Mar. 26, 2025 Mar. 26, 2025            
Principal/Shares $ 2,930                      
Cost 2,904                      
Market Value $ 2,648                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: WU Holdco, Inc. First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   WU Holdco, Inc.                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Consumer Goods Non Durable Sector [Member]         Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]
Spread Above Index (as a percent)   5.50%         5.50% 5.50% 5.50% 5.50% 5.50% 5.50%
Interest rate   6.50%         6.50% 6.50% 6.50% 6.50% 6.50% 6.50%
Maturity date   Mar. 26, 2026         Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026
Principal/Shares   $ 6,594                    
Cost   6,534                    
Market Value   $ 6,594                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: WU Holdco, Inc. First Lien Senior Secured Loan Libor One                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   WU Holdco, Inc.                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Consumer Goods Non Durable Sector [Member]         Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member] Consumer Goods Non Durable Sector [Member]
Maturity date   Mar. 26, 2026         Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026 Mar. 26, 2026
Cost   $ (31)                    
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Walker Edison First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] New Look Vision Group [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Retail Sector [Member]   Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
PIK (3.00%)   (3.00%) (3.00%) (3.00%) (3.00%)            
Interest rate 12.42%   12.42% 12.42% 12.42% 12.42%            
Maturity date Mar. 31, 2027   Mar. 31, 2027 Mar. 31, 2027 Mar. 31, 2027 Mar. 31, 2027            
Principal/Shares $ 20,912                      
Cost 20,736                      
Market Value $ 14,220                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: Walker Edison Initial Term Loan First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:WalkerEdisonInitialTermLoanMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Retail Sector [Member]         Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member] Retail Sector [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   9.75%         9.75% 9.75% 9.75% 9.75% 9.75% 9.75%
Maturity date   Aug. 05, 2027         Aug. 05, 2027 Aug. 05, 2027 Aug. 05, 2027 Aug. 05, 2027 Aug. 05, 2027 Aug. 05, 2027
Principal/Shares   $ 20,447                    
Cost   20,248                    
Market Value   $ 19,627                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: Whitcraft LLC First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Whitcraft L L C [Member] Whitcraft L L C [Member]                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] bcsf:FirstLienSeniorSecuredLoanRevolverMember First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 6.00%   6.00% 6.00% 6.00% 6.00%            
Interest rate 9.67%   9.67% 9.67% 9.67% 9.67%            
Maturity date Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023
Principal/Shares $ 28,760                      
Cost 28,705                      
Cost   $ (7)                    
Market Value $ 27,466 $ (59)                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Whitcraft LLC Senior Secured Loan First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] Whitcraft L L C [Member] Whitcraft L L C [Member]                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember First Lien Senior Secured Loan [Member] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member] Aerospace and Defense Sector [Member]
Spread Above Index (as a percent) 5.00% 6.00% 5.00% 5.00% 5.00% 5.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate 11.25% 7.00% 11.25% 11.25% 11.25% 11.25% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023 Apr. 03, 2023
Principal/Shares $ 254 $ 39,775                    
Cost 251 39,594                    
Market Value $ 172 $ 38,482                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] us-gaap:PrimeRateMember London Interbank Offered Rate (LIBOR) [Member] us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember us-gaap:PrimeRateMember London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: World Insurance First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   World Insurance [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Fire Insurance [Member]         Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date   Apr. 01, 2026         Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026
Principal/Shares   $ 3,144                    
Cost   3,088                    
Market Value   $ 3,121                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: World Insurance First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   World Insurance [Member]                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan Delayed Draw [Member]         First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration]   Fire Insurance [Member]         Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date   Apr. 01, 2026         Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026
Principal/Shares   $ 8,358                    
Cost   8,285                    
Market Value   $ 8,296                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: World Insurance First Lien Senior Secured Loan - Delayed Draw Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] World Insurance [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member]   First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member]   Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 9.30%   9.30% 9.30% 9.30% 9.30%            
Maturity date Apr. 01, 2026   Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026            
Principal/Shares $ 8,295                      
Cost 8,236                      
Market Value $ 8,295                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: World Insurance First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   World Insurance [Member]                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Fire Insurance [Member]         Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]
Spread Above Index (as a percent)   5.75%         5.75% 5.75% 5.75% 5.75% 5.75% 5.75%
Interest rate   6.75%         6.75% 6.75% 6.75% 6.75% 6.75% 6.75%
Maturity date   Apr. 01, 2026         Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026
Principal/Shares   $ 70                    
Cost   54                    
Market Value   $ 63                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor   1.00%         1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: World Insurance First Lien Senior Secured Loan - Revolver Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] World Insurance [Member]                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member]   Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 9.30%   9.30% 9.30% 9.30% 9.30%            
Maturity date Apr. 01, 2026   Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026            
Principal/Shares $ 256                      
Cost 243                      
Market Value $ 256                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: World Insurance First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] World Insurance [Member]                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Fire Insurance [Member]   Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member] Fire Insurance [Member]            
Spread Above Index (as a percent) 5.75%   5.75% 5.75% 5.75% 5.75%            
Interest rate 9.30%   9.30% 9.30% 9.30% 9.30%            
Maturity date Apr. 01, 2026   Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026 Apr. 01, 2026            
Principal/Shares $ 3,122                      
Cost 3,075                      
Market Value $ 3,122                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: YLG Holdings, Inc. First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] YLG Holdings, Inc. YLG Holdings, Inc.                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Spread Above Index (as a percent) 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25% 5.25%
Interest rate 7.10% 6.25% 7.10% 7.10% 7.10% 7.10% 6.25% 6.25% 6.25% 6.25% 6.25% 6.25%
Maturity date Oct. 31, 2025   Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025            
Principal/Shares $ 27,221 $ 38,086                    
Cost 27,124 37,900                    
Market Value $ 27,153 $ 38,086                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: YLG Holdings, Inc. First Lien Senior Secured Loan - Delayed Draw                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] YLG Holdings, Inc. YLG Holdings, Inc.                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member] First Lien Senior Secured Loan Delayed Draw [Member]
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Spread Above Index (as a percent) 5.00% 6.00% 5.00% 5.00% 5.00% 5.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate 7.08% 7.00% 7.08% 7.08% 7.08% 7.08% 7.00% 7.00% 7.00% 7.00% 7.00% 7.00%
Maturity date Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025
Principal/Shares $ 5,022 $ 5,060                    
Cost 5,017 5,055                    
Market Value $ 5,009 $ 5,060                    
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Interest Rate, Floor 0.25% 1.00% 0.25% 0.25% 0.25% 0.25% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: YLG Holdings, Inc. First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] YLG Holdings, Inc. YLG Holdings, Inc.                    
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] Construction Sector [Member] bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember bcsf:ConstructionAndBuildingSectorMember
Spread Above Index (as a percent) 5.00%   5.00% 5.00% 5.00% 5.00%            
Interest rate 7.93%   7.93% 7.93% 7.93% 7.93%            
Maturity date Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025 Oct. 31, 2025
Principal/Shares $ 855                      
Cost 811                      
Cost   $ (55)                    
Market Value $ 833                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00% 1.00%
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: Zeppelin BidCo Pty Limited First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration] bcsf:ZeppelinBidCoPtyLimitedMember bcsf:ZeppelinBidCoPtyLimitedMember                    
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member] Services, Consumer [Member]
Spread Above Index (as a percent) 5.00% 6.00% 5.00% 5.00% 5.00% 5.00% 6.00% 6.00% 6.00% 6.00% 6.00% 6.00%
Interest rate 7.12% 5.12% 7.12% 7.12% 7.12% 7.12% 5.12% 5.12% 5.12% 5.12% 5.12% 5.12%
Maturity date Jun. 28, 2024 Jun. 28, 2024 Jun. 28, 2024 Jun. 28, 2024 Jun. 28, 2024 Jun. 28, 2024 Jun. 28, 2024 Jun. 28, 2024 Jun. 28, 2024 Jun. 28, 2024 Jun. 28, 2024 Jun. 28, 2024
Principal/Shares     $ 206       $ 206          
Cost $ 142           142          
Market Value $ 131           $ 150          
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember bcsf:BankBillSwapBidRateMember
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false false false false false false false false false false false false
Investment, Significant Unobservable Input [true false] true true true true true true true true true true true true
Investment, Non-income Producing [true false] false false false false false false false false false false false false
Investment, Identifier [Axis]: iBanFirst Facility B First Lien Senior Secured Loan Euibor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:iBanFirstFacilityBMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   8.50%         8.50% 8.50% 8.50% 8.50% 8.50% 8.50%
Interest rate   10.00%         10.00% 10.00% 10.00% 10.00% 10.00% 10.00%
Maturity date   Jul. 13, 2028         Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028
Principal/Shares | €               € 102        
Cost | €               128        
Market Value | €               € 116        
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:EuroInterbankOfferRateMember         bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: iBanFirst Facility Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:iBanFirstFacilityMember                      
Investment, Type [Extensible Enumeration] Preferred Stock [Member]   Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Principal/Shares | €       € 7,112                
Cost $ 8,136                      
Market Value $ 9,252                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] true   true true true true            
Investment, Identifier [Axis]: iBanFirst Facility Series A Preferred Units Preferred Equity                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:iBanFirstFacilitySeriesAPreferredUnitsMember                    
Investment, Type [Extensible Enumeration]   Preferred Stock [Member]         Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member] Preferred Stock [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Principal/Shares   $ 5,080                    
Cost   5,996                    
Market Value   $ 6,290                    
Acquisition date Jul. 13, 2021 Jul. 13, 2021                    
Investment, Restriction Status [true false]   true         true true true true true true
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   true         true true true true true true
Investment, Identifier [Axis]: iBanFirst First Lien Senior Secured Loan 1                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:iBanFirstMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 8.50%   8.50% 8.50% 8.50% 8.50%            
Interest rate 10.00%   10.00% 10.00% 10.00% 10.00%            
Maturity date Jul. 13, 2028   Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028            
Principal/Shares $ 2,748                      
Cost 2,817                      
Market Value $ 2,694                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 1.50%   1.50% 1.50% 1.50% 1.50%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: iBanFirst First Lien Senior Secured Loan 2                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:iBanFirstMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 10.00%   10.00% 10.00% 10.00% 10.00%            
Interest rate 10.00%   10.00% 10.00% 10.00% 10.00%            
Maturity date Jul. 13, 2028   Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028            
Principal/Shares | €       € 80                
Cost $ 83                      
Market Value $ 78                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] bcsf:EuroInterbankOfferRateMember   bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember            
Investment, Interest Rate, Floor 0.00%   0.00% 0.00% 0.00% 0.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: iBanFirst First Lien Senior Secured Loan Euribor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:iBanFirstMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Maturity date Jul. 13, 2028   Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: iBanFirst Revolving Facility First Lien Senior Secured Loan - Revolver Euibor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:iBanFirstRevolvingFacilityMember                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   8.50%         8.50% 8.50% 8.50% 8.50% 8.50% 8.50%
Interest rate   8.50%         8.50% 8.50% 8.50% 8.50% 8.50% 8.50%
Maturity date   Jul. 13, 2028         Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028 Jul. 13, 2028
Principal/Shares | €               € 2,030        
Cost | €               2,244        
Market Value | €               € 2,308        
Investment, Variable Interest Rate, Type [Extensible Enumeration]   bcsf:EuroInterbankOfferRateMember         bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember bcsf:EuroInterbankOfferRateMember
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: masLabor Equity Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:masLaborEquityMember                    
Investment, Type [Extensible Enumeration]   Common Stock [Member]         Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Principal/Shares   $ 345                    
Cost   345                    
Market Value   $ 372                    
Acquisition date   Jul. 01, 2021                    
Investment, Restriction Status [true false]   true         true true true true true true
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   true         true true true true true true
Investment, Identifier [Axis]: masLabor Equity Interest                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:masLaborMember                      
Investment, Type [Extensible Enumeration] Common Stock [Member]   Common Stock [Member] Common Stock [Member] Common Stock [Member] Common Stock [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Principal/Shares $ 345                      
Cost 345                      
Market Value $ 909                      
Acquisition date Jul. 01, 2021                      
Investment, Restriction Status [true false] true   true true true true            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Identifier [Axis]: masLabor First Lien Senior Secured Loan                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:masLaborMember                      
Investment, Type [Extensible Enumeration] First Lien Senior Secured Loan [Member]   First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Spread Above Index (as a percent) 7.50%   7.50% 7.50% 7.50% 7.50%            
Interest rate 8.50%   8.50% 8.50% 8.50% 8.50%            
Maturity date Jul. 01, 2027   Jul. 01, 2027 Jul. 01, 2027 Jul. 01, 2027 Jul. 01, 2027            
Principal/Shares | £         £ 8,513              
Cost $ 8,289                      
Market Value $ 8,513                      
Investment, Variable Interest Rate, Type [Extensible Enumeration] London Interbank Offered Rate (LIBOR) [Member]   London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]            
Investment, Interest Rate, Floor 1.00%   1.00% 1.00% 1.00% 1.00%            
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: masLabor First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration] Investment, Unaffiliated Issuer [Member]   Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]            
Investment, Issuer Name [Extensible Enumeration] bcsf:masLaborMember                      
Investment, Type [Extensible Enumeration] bcsf:FirstLienSeniorSecuredLoanRevolverMember   bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember            
Investment, Industry Sector [Extensible Enumeration] Services, Business [Member]   Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]            
Maturity date Jul. 01, 2027   Jul. 01, 2027 Jul. 01, 2027 Jul. 01, 2027 Jul. 01, 2027            
Cost $ (18)                      
Investment, Restriction Status [true false] false   false false false false            
Investment, Significant Unobservable Input [true false] true   true true true true            
Investment, Non-income Producing [true false] false   false false false false            
Investment, Identifier [Axis]: masLabor Revolver First Lien Senior Secured Loan - Revolver                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:masLaborRevolverMember                    
Investment, Type [Extensible Enumeration]   bcsf:FirstLienSeniorSecuredLoanRevolverMember         bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember bcsf:FirstLienSeniorSecuredLoanRevolverMember
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Maturity date   Jul. 01, 2027         Jul. 01, 2027 Jul. 01, 2027 Jul. 01, 2027 Jul. 01, 2027 Jul. 01, 2027 Jul. 01, 2027
Cost   $ (21)                    
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false
Investment, Identifier [Axis]: masLabor Term Loan Note First Lien Senior Secured Loan Libor                        
Consolidated Schedule of Investments                        
Investment, Issuer Affiliation [Extensible Enumeration]   Investment, Unaffiliated Issuer [Member]         Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member] Investment, Unaffiliated Issuer [Member]
Investment, Issuer Name [Extensible Enumeration]   bcsf:masLaborTermLoanNoteMember                    
Investment, Type [Extensible Enumeration]   First Lien Senior Secured Loan [Member]         First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member] First Lien Senior Secured Loan [Member]
Investment, Industry Sector [Extensible Enumeration]   Services, Business [Member]         Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member] Services, Business [Member]
Spread Above Index (as a percent)   7.50%         7.50% 7.50% 7.50% 7.50% 7.50% 7.50%
Interest rate   8.50%         8.50% 8.50% 8.50% 8.50% 8.50% 8.50%
Maturity date   Jul. 01, 2027         Jul. 01, 2027 Jul. 01, 2027 Jul. 01, 2027 Jul. 01, 2027 Jul. 01, 2027 Jul. 01, 2027
Principal/Shares   $ 8,578                    
Cost   8,324                    
Market Value   $ 8,578                    
Investment, Variable Interest Rate, Type [Extensible Enumeration]   London Interbank Offered Rate (LIBOR) [Member]         London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member] London Interbank Offered Rate (LIBOR) [Member]
Investment, Restriction Status [true false]   false         false false false false false false
Investment, Significant Unobservable Input [true false]   true         true true true true true true
Investment, Non-income Producing [true false]   false         false false false false false false