Fair Value of Financial Instruments - Investments at Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2022		Dec. 31, 2021
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at Amortized Cost	$ 2,927,131	[1]	$ 2,783,617	[2]
Fair Value	2,875,144	[1]	$ 2,778,964	[2]
Fair Value Percentage of Total Portfolio			100.00%
FBLC Senior Loan Fund, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at Amortized Cost	923,304		$ 1,085,170
Fair Value	$ 855,705		$ 1,088,337
Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	100.00%		100.00%
Senior Secured First Lien Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at Amortized Cost	$ 2,048,997		$ 1,865,403
Fair Value	1,995,273		1,831,170
Senior Secured First Lien Debt | FBLC Senior Loan Fund, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at Amortized Cost	809,938		945,196
Fair Value	$ 754,019		$ 947,148
Senior Secured First Lien Debt | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	69.40%		65.90%
Senior Secured Second Lien Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at Amortized Cost	$ 223,193		$ 251,149
Fair Value	203,299		243,932
Senior Secured Second Lien Debt | FBLC Senior Loan Fund, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at Amortized Cost	28,577		61,932
Fair Value	$ 26,570		$ 62,086
Senior Secured Second Lien Debt | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	7.10%		8.80%
Subordinated Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at Amortized Cost	$ 134,742		$ 117,627
Fair Value	$ 134,958		$ 118,077
Subordinated Debt | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	4.70%		4.20%
Collateralized Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at Amortized Cost	$ 48,088		$ 51,083
Fair Value	30,288		37,097
Collateralized Securities | FBLC Senior Loan Fund, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at Amortized Cost	84,789		78,042
Fair Value	$ 75,116		$ 79,103
Collateralized Securities | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	1.10%		1.30%
Equity/Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at Amortized Cost	$ 167,177		$ 193,421
Fair Value	206,392		243,754
Equity/Other | FBLC Senior Loan Fund, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at Amortized Cost			304,934
Fair Value	$ 304,934		$ 304,934
Equity/Other | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	7.10%		8.80%
Equity/Other | Investment Owned, At Fair Value | Investment Type Concentration Risk | FBLC Senior Loan Fund, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio			11.00%
FBLC Senior Loan Fund, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Investments at Amortized Cost	$ 304,934
Fair Value	$ 304,934
FBLC Senior Loan Fund, LLC | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	10.60%

[1] Percentages are based on net assets as of December 31, 2022.

[2] Percentages are based on net assets as of December 31, 2021.