Investment Portfolio - Investment Portfolio, at Fair Value and Percentage (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2022		Dec. 31, 2021
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 15,791.8	[1]	$ 16,101.0
Investment Owned, At Fair Value | 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	$ 36.0		$ 89.0
Automobiles & Components | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.20%		0.50%
Banks
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 0.0		$ 15.0
Banks | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.00%		0.10%
Capital Goods
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 2,224.0		$ 2,281.0
Capital Goods | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	14.10%		14.20%
Commercial & Professional Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 1,673.0		$ 1,615.0
Commercial & Professional Services | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	10.60%		10.00%
Media & Entertainment
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 290.0		$ 551.0
Media & Entertainment | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.80%		3.40%
Pharmaceuticals, Biotechnology & Life Sciences
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 200.0		$ 393.0
Pharmaceuticals, Biotechnology & Life Sciences | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.30%		2.40%
Credit Opportunities Partners JV, LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 1,466.0		$ 1,396.0
Credit Opportunities Partners JV, LLC | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	9.30%		8.70%
Diversified Financials
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 634.0		$ 672.0
Diversified Financials | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	4.00%		4.20%
Energy
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 288.0		$ 241.0
Energy | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.80%		1.50%
Software & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 268.0		$ 296.0
Software & Services | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.70%		1.80%
Food, Beverage & Tobacco
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 189.0		$ 256.0
Food, Beverage & Tobacco | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.20%		1.60%
Health Care Equipment & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 1,976.0		$ 1,613.0
Health Care Equipment & Services | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	12.50%		10.00%
Health Care Equipment & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 246.0		$ 227.0
Health Care Equipment & Services | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.60%		1.40%
Insurance
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 950.0		$ 898.0
Insurance | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	6.00%		5.60%
Food & Staples Retailing
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 190.0		$ 211.0
Food & Staples Retailing | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.20%		1.30%
Technology Hardware & Equipment
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 627.0		$ 720.0
Technology Hardware & Equipment | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	4.00%		4.50%
Insurance
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 202.0		$ 235.0
Insurance | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.30%		1.50%
Materials
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 818.0		$ 876.0
Materials | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	5.20%		5.40%
Technology Hardware & Equipment
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 287.0		$ 288.0
Technology Hardware & Equipment | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.80%		1.80%
Software & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 2,847.0		$ 2,698.0
Software & Services | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	18.00%		16.80%
Health Care Equipment & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 5.0		$ 42.0
Health Care Equipment & Services | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.00%		0.30%
Telecommunication Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 79.0		$ 128.0
Telecommunication Services | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.50%		0.80%
Transportation
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 297.0		$ 360.0
Transportation | Investment Owned, At Fair Value | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.90%		2.20%

[1] Security may be an obligation of one or more entities affiliated with the named company