Investment Portfolio - Investment Portfolio, at Fair Value and Percentage (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2022		Dec. 31, 2021
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 15,377.3	[1]	$ 16,101.5	[2]
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]
Investments, at fair value	$ 25.0		$ 89.0
Automobiles & Components | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.20%		0.50%
Banks
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 0.0		$ 15.0
Banks | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.00%		0.10%
Capital Goods
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 2,366.0		$ 2,281.0
Capital Goods | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	15.40%		14.20%
Commercial & Professional Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 1,670.0		$ 1,615.0
Commercial & Professional Services | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	10.90%		10.00%
Consumer Durables & Apparel
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 235.0		$ 551.0
Consumer Durables & Apparel | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.50%		3.40%
Consumer Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 189.0		$ 393.0
Consumer Services | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.20%		2.40%
Credit Opportunities Partners JV, LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 1,428.0		$ 1,396.0
Credit Opportunities Partners JV, LLC | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	9.30%		8.70%
Diversified Financials
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 583.0		$ 672.0
Diversified Financials | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	3.80%		4.20%
Energy
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 272.0		$ 241.0
Energy | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.80%		1.50%
Food & Staples Retailing
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 103.0		$ 296.0
Food & Staples Retailing | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.70%		1.80%
Food, Beverage & Tobacco
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 226.0		$ 256.0
Food, Beverage & Tobacco | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.50%		1.60%
Health Care Equipment & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 1,963.0		$ 1,613.0
Health Care Equipment & Services | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	12.80%		10.00%
Household & Personal Products
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 242.0		$ 227.0
Household & Personal Products | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.60%		1.40%
Insurance
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 974.0		$ 898.0
Insurance | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	6.30%		5.60%
Materials
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 197.0		$ 211.0
Materials | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.30%		1.30%
Media & Entertainment
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 695.0		$ 720.0
Media & Entertainment | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	4.50%		4.50%
Pharmaceuticals, Biotechnology & Life Sciences
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 231.0		$ 235.0
Pharmaceuticals, Biotechnology & Life Sciences | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.40%		1.50%
Real Estate
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 753.0		$ 876.0
Real Estate | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	4.90%		5.40%
Retailing
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 282.0		$ 288.0
Retailing | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.80%		1.80%
Software & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 2,591.0		$ 2,698.0
Software & Services | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	16.80%		16.80%
Technology Hardware & Equipment
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 1.0		$ 42.0
Technology Hardware & Equipment | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.00%		0.30%
Telecommunication Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 76.0		$ 128.0
Telecommunication Services | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.50%		0.80%
Transportation
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 275.0		$ 360.0
Transportation | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.80%		2.20%

[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).