Debt - 2019 1 Debt (Details)				12 Months Ended
	Jun. 15, 2023	Nov. 30, 2021 USD ($)	Aug. 28, 2019 USD ($)	Dec. 31, 2023 USD ($) Loan	Dec. 31, 2022 USD ($) Loan	Dec. 31, 2021 USD ($)
Debt
Principal amount				$ 1,617,500,000	$ 1,667,500,000
Net realized loss on extinguishment of debt					$ (747,000)	$ (4,859,000)
Number of investments secured as collateral | Loan				49	49
Fair value of investments secured as collateral				$ 453,700,000	$ 447,400,000
Cash secured as collateral				52,800,000	56,000,000
Unamortized debt issuance costs				7,567,000	10,197,000
2019-1 Notes
Debt
Realized gains (losses) on debt securitization			$ 0
Principal amount		$ 410,000,000	$ 501,000,000	352,500,000	352,500,000
Net realized loss on extinguishment of debt		$ 2,300,000
Membership interest retained (as a percent)		5.00%
Unamortized debt issuance costs				1,300,000	$ 1,400,000	$ 1,500,000
2019-1 Notes | BCC Middle Market CLO 2019 | SOFR
Debt
Spread rate (as a percent)	0.26%
Class A1R senior secured floating rate notes
Debt
Principal amount		$ 282,500,000		$ 282,500,000
Interest rate (as a percent)		1.50%		7.16%
Spread rate (as a percent)				1.50%
Class A2R senior secured floating rate notes
Debt
Principal amount		$ 55,000,000		$ 55,000,000
Interest rate (as a percent)		2.00%		7.66%
Spread rate (as a percent)				2.00%
Class BR senior deferrable floating rate notes
Debt
Principal amount		$ 47,500,000		$ 15,000,000
Interest rate (as a percent)		2.60%		8.26%
Debt instrument retained		$ 32,500,000
Spread rate (as a percent)				2.60%
Class CR senior deferrable floating rate notes
Debt
Principal amount		$ 25,000,000
Interest rate (as a percent)		3.75%
Debt instrument retained		$ 25,000,000