Investment Portfolio - Investment Portfolio, at Fair Value and Percentage (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2023		Dec. 31, 2022
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 14,666.1	[1]	$ 15,377.3	[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]
Fair Value	$ 4.0		$ 25.0
Automobiles & Components | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.00%		0.20%
Banks
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 10.0		$ 0.0
Banks | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.10%		0.00%
Capital Goods
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 2,241.0		$ 2,366.0
Capital Goods | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	15.30%		15.40%
Commercial & Professional Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 1,761.0		$ 1,670.0
Commercial & Professional Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	12.00%		10.90%
Consumer Discretionary Distribution & Retail
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 238.0		$ 282.0
Consumer Discretionary Distribution & Retail | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.60%		1.80%
Consumer Durables & Apparel
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 191.0		$ 235.0
Consumer Durables & Apparel | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.30%		1.50%
Consumer Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 206.0		$ 189.0
Consumer Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.40%		1.20%
Consumer Staples Distribution & Retail
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 109.0		$ 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	$ 1,403.0		$ 1,428.0
Credit Opportunities Partners JV, LLC | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	9.60%		9.30%
Energy
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 211.0		$ 272.0
Energy | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.40%		1.80%
Equity Real Estate Investment Trusts (REITs)
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 288.0		$ 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	$ 816.0		$ 844.0
Financial Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	5.60%		5.50%
Food, Beverage & Tobacco
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 200.0		$ 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	$ 1,774.0		$ 1,963.0
Health Care Equipment & Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	12.10%		12.80%
Household & Personal Products
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 166.0		$ 242.0
Household & Personal Products | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.10%		1.60%
Insurance
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 829.0		$ 974.0
Insurance | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	5.60%		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	$ 716.0		$ 695.0
Media & Entertainment | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	4.90%		4.50%
Pharmaceuticals, Biotechnology & Life Sciences
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 293.0		$ 231.0
Pharmaceuticals, Biotechnology & Life Sciences | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	2.00%		1.50%
Real Estate Management & Development
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 97.0		$ 156.0
Real Estate Management & Development | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.70%		1.00%
Software & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 2,589.0		$ 2,591.0
Software & Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	17.70%		16.80%
Technology Hardware & Equipment
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 3.0		$ 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	$ 74.0		$ 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	$ 220.0		$ 275.0
Transportation | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.50%		1.80%

[1] See Note 8 for additional information regarding the fair value of the Company’s financial instruments

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