Investment Portfolio - Balance Sheet Information for COPJV (Details) - USD ($) $ in Millions	Dec. 31, 2022		Dec. 31, 2021		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 15,377.3	[1]	$ 16,101.5	[2]
Total assets	16,124.0		17,228.0
Total liabilities	9,112.0		9,498.0
Total stockholders' equity	7,012.0		7,730.0		$ 3,096.0	$ 3,866.0	$ 4,166.0
Credit Opportunities Partners JV, LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	3,562.9		3,260.0
Cash and other assets	259.2		414.3
Total assets	3,822.1		3,674.3
Debt	1,913.4		1,442.0
Other liabilities	276.4		636.7
Total liabilities	2,189.8		2,078.7
Total stockholders' equity	$ 1,632.3		$ 1,595.6

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

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