Investments - SLP (Details) - USD ($) $ in Thousands							3 Months Ended
	Mar. 13, 2024	Sep. 27, 2023	Jun. 15, 2023	Aug. 24, 2022	Mar. 07, 2022	Feb. 09, 2022	Mar. 31, 2024	Mar. 31, 2023	Dec. 31, 2023	Aug. 09, 2023
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value							$ 2,405,952		$ 2,298,343
Investments sold							1,032,100
Proceeds from principal payments and sales of investments							296,739	$ 252,880
Total aggregate principal amount committed							$ 1,617,500		$ 1,617,500
Weighted average interest rate of debt (as a percent)							5.20%		5.20%
Subordinated Debt [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value							$ 46,618		$ 45,877
Preferred Stock [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value							$ 107,166		$ 104,428
2018-1 Notes
Investment Company, Nonconsolidated Subsidiary [Line Items]
Net realized loss on extinguishment of debt	$ (1,300)
2023-1 Notes
Investment Company, Nonconsolidated Subsidiary [Line Items]
Term Debt Secuiritisation										$ 400,000
Class A-2-R
Investment Company, Nonconsolidated Subsidiary [Line Items]
Spread rate (as a percent)	2.90%
Total aggregate principal amount committed	$ 30,000
BCC Middle Market CLO 20181 LLC | 2018-1 Notes | SOFR
Investment Company, Nonconsolidated Subsidiary [Line Items]
Spread rate (as a percent)			0.26%
Bain Capital Senior Loan Program, LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Total aggregate principal amount committed	500,000
Weighted average interest rate of debt (as a percent)							8.50%		7.70%
Bain Capital Senior Loan Program, LLC | M M 22 2 Credit Facility [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Maximum borrowing capacity				$ 225,000
Bain Capital Senior Loan Program, LLC | M M 22 2 Credit Facility [Member] | SOFR
Investment Company, Nonconsolidated Subsidiary [Line Items]
Spread rate (as a percent)				2.10%
Bain Capital Senior Loan Program, LLC | M M 23 3 Credit Facility [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Maximum borrowing capacity		$ 140,000
Outstanding balance							$ 90,900		$ 97,900
Effective interest rate							8.10%		8.20%
Bain Capital Senior Loan Program, LLC | M M 23 3 Credit Facility [Member] | SOFR
Investment Company, Nonconsolidated Subsidiary [Line Items]
Spread rate (as a percent)		2.85%
Bain Capital Senior Loan Program, LLC | Subordinated Debt [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value							$ 119,000		$ 116,000
Bain Capital Senior Loan Program, LLC | Preferred Stock [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value							(2,300)		(1,800)
Bain Capital Senior Loan Program, LLC | Common Stock [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value							$ (1,800)		$ 400
Bain Capital Senior Loan Program, LLC | 2018-1 Notes | Membership Interests [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Total aggregate principal amount committed	$ 22,400
Bain Capital Senior Loan Program, LLC | Class A-1-R
Investment Company, Nonconsolidated Subsidiary [Line Items]
Spread rate (as a percent)	2.25%
Total aggregate principal amount committed	$ 290,000
Bain Capital Senior Loan Program, LLC | Class A-J-R
Investment Company, Nonconsolidated Subsidiary [Line Items]
Spread rate (as a percent)	2.70%
Total aggregate principal amount committed	$ 20,000
Bain Capital Senior Loan Program, LLC | Class B-R
Investment Company, Nonconsolidated Subsidiary [Line Items]
Spread rate (as a percent)	3.90%
Total aggregate principal amount committed	$ 40,000
Bain Capital Senior Loan Program, LLC | Class C-R
Investment Company, Nonconsolidated Subsidiary [Line Items]
Spread rate (as a percent)	5.90%
Total aggregate principal amount committed	$ 30,000
Bain Capital Senior Loan Program, LLC | Class D-R
Investment Company, Nonconsolidated Subsidiary [Line Items]
Spread rate (as a percent)	8.32%
Total aggregate principal amount committed	$ 30,000
Bain Capital Senior Loan Program, LLC | BCC Middle Market CLO 20181 LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Membership equity interest acquired					70.00%	70.00%
Membership interests (as a percent)					30.00%	30.00%
Proceeds from principal payments and sales of investments					$ 56,100
Net realized gain from sale of investment					$ 1,200
Amberstone Co., Ltd. | Bain Capital Senior Loan Program, LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Economic ownership interest						50.00%
Initial capital contributions						$ 179,000