Investment Portfolio - Balance Sheet Information for COPJV (Details) - USD ($) $ in Millions	Mar. 31, 2023	Dec. 31, 2022		Mar. 31, 2022	Dec. 31, 2021
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 15,277.1	$ 15,377.3	[1]
Total assets	16,058.0	16,124.0
Total liabilities	9,075.0	9,112.0
Member's equity	6,983.0	7,012.0		$ 7,767.0	$ 7,730.0
Credit Opportunities Partners JV, LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	3,556.3	3,562.9
Cash	256.2	259.2
Total assets	3,812.5	3,822.1
Debt	1,936.0	1,913.4
Other liabilities	280.4	276.4
Total liabilities	2,216.4	2,189.8
Member's equity	$ 1,596.1	$ 1,632.3

[1] Fair value determined by the Company’s board of directors (see Note 8).