Investment Portfolio - Balance Sheet Information for COPJV (Details) - USD ($) $ in Millions	Mar. 31, 2024		Dec. 31, 2023	Mar. 31, 2023	Dec. 31, 2022
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 14,220.8	[1]	$ 14,649.0
Total assets	15,152.0		15,469.0
Total liabilities	8,340.0		8,620.0
Member's equity	6,812.0		6,849.0	$ 6,983.0	$ 7,012.0
Credit Opportunities Partners JV, LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	3,403.6		3,470.8
Cash and other assets	354.2		272.2
Total assets	3,757.8		3,743.0
Debt	1,799.6		1,840.1
Other liabilities	369.2		306.5
Total liabilities	2,168.8		2,146.6
Member's equity	$ 1,589.0		$ 1,596.4

[1] See Note 8 for additional information regarding the fair value of the Company’s financial instruments