Investment Portfolio - Investment Portfolio, at Fair Value and Percentage (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024		Dec. 31, 2023
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 13,490.4	[1],[2]	$ 14,649.3	[3],[4]
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	$ 4.0		$ 4.0
Automobiles & Components | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.00%		0.00%
Banks
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 5.0		$ 8.0
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	$ 1,712.0		$ 1,970.0
Capital Goods | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	12.70%		13.50%
Commercial & Professional Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 1,733.0		$ 1,826.0
Commercial & Professional Services | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	12.80%		12.50%
Consumer Discretionary Distribution & Retail
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 174.0		$ 259.0
Consumer Discretionary Distribution & Retail | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.30%		1.80%
Consumer Durables & Apparel
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 229.0		$ 185.0
Consumer Durables & Apparel | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.70%		1.30%
Consumer Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 244.0		$ 240.0
Consumer Services | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.80%		1.60%
Consumer Staples Distribution & Retail
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 102.0		$ 105.0
Consumer Staples Distribution & Retail | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.80%		0.70%
Credit Opportunities Partners JV, LLC
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 1,363.0		$ 1,397.0
Credit Opportunities Partners JV, LLC | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	10.10%		9.50%
Energy
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 89.0		$ 162.0
Energy | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.70%		1.10%
Equity Real Estate Investment Trusts (REITs)
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 278.0		$ 293.0
Equity Real Estate Investment Trusts (REITs) | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	2.10%		2.00%
Financial Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 998.0		$ 986.0
Financial Services | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	7.40%		6.70%
Food, Beverage & Tobacco
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 113.0		$ 181.0
Food, Beverage & Tobacco | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.80%		1.20%
Health Care Equipment & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 1,667.0		$ 1,709.0
Health Care Equipment & Services | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	12.40%		11.70%
Household & Personal Products
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 134.0		$ 166.0
Household & Personal Products | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.00%		1.10%
Insurance
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 735.0		$ 839.0
Insurance | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	5.40%		5.70%
Materials
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 334.0		$ 228.0
Materials | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	2.50%		1.60%
Media & Entertainment
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 699.0		$ 717.0
Media & Entertainment | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	5.20%		4.90%
Pharmaceuticals, Biotechnology & Life Sciences
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 298.0		$ 286.0
Pharmaceuticals, Biotechnology & Life Sciences | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	2.20%		2.00%
Real Estate Management & Development
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 27.0		$ 79.0
Real Estate Management & Development | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.20%		0.50%
Software & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 2,187.0		$ 2,472.0
Software & Services | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	16.20%		16.90%
Technology Hardware & Equipment
Investment Company, Nonconsolidated Subsidiary [Line Items]
Investments, at fair value	$ 2.0		$ 4.0
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]
Investments, at fair value	$ 69.0		$ 79.0
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]
Investments, at fair value	$ 294.0		$ 454.0
Transportation | Percentage of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	2.20%		3.10%

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

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

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

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