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Investments - SLP (Details) - USD ($)				6 Months Ended
Investments - SLP (Details) - USD ($)	Jun. 15, 2023	Mar. 07, 2022	Feb. 09, 2022	Jun. 30, 2023	Jun. 30, 2022	Dec. 31, 2022
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value				$ 2,385,272,000		$ 2,386,977,000
Investments sold				917.6
Proceeds from principal payments and sales of investments				$ 435,472,000	$ 434,198,000
Weighted average interest rate of debt (as a percent)				5.10%		3.50%
Subordinated Debt [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value				$ 44,446,000		$ 43,922,000
Preferred Stock [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value				$ 99,650,000		$ 80,945,000
BCC Middle Market CLO 20181 LLC \| 2018-1 Notes \| SOFR
Investment Company, Nonconsolidated Subsidiary [Line Items]
Spread rate (as a percent)	0.26161%
Bain Capital Senior Loan Program, LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Economic ownership interest			50.00%
Weighted average interest rate of debt (as a percent)				6.90%		4.30%
Bain Capital Senior Loan Program, LLC \| M M 22 2 Credit Facility [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Maximum borrowing capacity				$ 225,000,000
Outstanding balance				$ 219,800,000		$ 113,700,000
Effective interest rate				7.20%		6.40%
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 \| Subordinated Debt [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value				$ 116,000,000		$ 51,000,000
Bain Capital Senior Loan Program, LLC \| Preferred Stock [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value				(1,100,000)		(600,000)
Bain Capital Senior Loan Program, LLC \| Common Stock [Member]
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments at fair value				$ 1,900,000		$ 3,300,000
Bain Capital Senior Loan Program, LLC \| BCC Middle Market CLO 20181 LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Membership interests (as a percent)		30.00%	30.00%
Proceeds from principal payments and sales of investments		$ 56,100,000
Net realized gain from sale of investment		$ 1,200,000
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%
Amberstone Co., Ltd. \| Bain Capital Senior Loan Program, LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Economic ownership interest			50.00%
Initial capital contributions			$ 179,000,000