Investment Portfolio - Investment Portfolio, at Fair Value and Percentage (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2023	Dec. 31, 2022
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 15,277.1	$ 15,377.3	[1]
Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	100.00%	100.00%
Automobiles & Components
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 8.0	$ 25.0
Automobiles & Components | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.10%	0.20%
Capital Goods
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 2,357.0	$ 2,366.0
Capital Goods | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	15.40%	15.40%
Commercial & Professional Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 1,776.0	$ 1,670.0
Commercial & Professional Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	11.60%	10.90%
Consumer Discretionary Distribution & Retail
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 282.0	$ 282.0
Consumer Discretionary Distribution & Retail | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.80%	1.80%
Consumer Durables & Apparel
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 239.0	$ 235.0
Consumer Durables & Apparel | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.60%	1.50%
Consumer Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 200.0	$ 189.0
Consumer Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.30%	1.20%
Consumer Staples Distribution & Retail
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 106.0	$ 103.0
Consumer Staples Distribution & Retail | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.70%	0.70%
Credit Opportunities Partners JV, LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 1,397.0	$ 1,428.0
Credit Opportunities Partners JV, LLC | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	9.10%	9.30%
Energy
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 230.0	$ 272.0
Energy | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.50%	1.80%
Equity Real Estate Investment Trusts (REITs)
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 307.0	$ 336.0
Equity Real Estate Investment Trusts (REITs) | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	2.00%	2.20%
Financial Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 801.0	$ 844.0
Financial Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	5.20%	5.50%
Food, Beverage & Tobacco
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 207.0	$ 226.0
Food, Beverage & Tobacco | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.40%	1.40%
Health Care Equipment & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 1,951.0	$ 1,963.0
Health Care Equipment & Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	12.80%	12.80%
Household & Personal Products
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 238.0	$ 242.0
Household & Personal Products | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.60%	1.60%
Insurance
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 969.0	$ 974.0
Insurance | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	6.30%	6.30%
Materials
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 227.0	$ 197.0
Materials | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.50%	1.30%
Media & Entertainment
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 730.0	$ 695.0
Media & Entertainment | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	4.80%	4.50%
Pharmaceuticals, Biotechnology & Life Sciences
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 240.0	$ 231.0
Pharmaceuticals, Biotechnology & Life Sciences | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.60%	1.50%
Real Estate Management & Development
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 130.0	$ 156.0
Real Estate Management & Development | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.90%	1.00%
Software & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 2,525.0	$ 2,591.0
Software & Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	16.50%	16.80%
Technology Hardware & Equipment
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 2.0	$ 1.0
Technology Hardware & Equipment | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.00%	0.00%
Telecommunication Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 73.0	$ 76.0
Telecommunication Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.50%	0.50%
Transportation
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 282.0	$ 275.0
Transportation | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.80%	1.80%

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