Investment Portfolio - Schedule of Investment Portfolio, at Fair Value and Percentage (Details) - USD ($) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2024		Dec. 31, 2023
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 13,943.4	[1],[2]	$ 14,649.3	[3],[4],[5],[6],[7]
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		$ 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]
Fair Value	$ 6.0		$ 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]
Fair Value	$ 1,855.0		$ 1,970.0
Capital Goods | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	13.30%		13.50%
Commercial & Professional Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 1,856.0		$ 1,826.0
Commercial & Professional Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	13.30%		12.50%
Consumer Discretionary Distribution & Retail
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 153.0		$ 259.0
Consumer Discretionary Distribution & Retail | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.10%		1.80%
Consumer Durables & Apparel
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 249.0		$ 185.0
Consumer Durables & Apparel | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.80%		1.30%
Consumer Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 372.0		$ 240.0
Consumer Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	2.70%		1.60%
Consumer Staples Distribution & Retail
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 103.0		$ 105.0
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	$ 1,384.0		$ 1,397.0
Credit Opportunities Partners JV, LLC | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	9.90%		9.50%
Energy
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 83.0		$ 162.0
Energy | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	0.60%		1.10%
Equity Real Estate Investment Trusts (REITs)
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 274.0		$ 293.0
Equity Real Estate Investment Trusts (REITs) | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	2.00%		2.00%
Financial Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 879.0		$ 986.0
Financial Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	6.30%		6.70%
Food, Beverage & Tobacco
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 144.0		$ 181.0
Food, Beverage & Tobacco | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	1.00%		1.20%
Health Care Equipment & Services
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 1,630.0		$ 1,709.0
Health Care Equipment & Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	11.70%		11.70%
Household & Personal Products
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 141.0		$ 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]
Fair Value	$ 805.0		$ 839.0
Insurance | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	5.80%		5.70%
Materials
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 335.0		$ 228.0
Materials | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	2.40%		1.60%
Media & Entertainment
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 731.0		$ 717.0
Media & Entertainment | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	5.30%		4.90%
Pharmaceuticals, Biotechnology & Life Sciences
Investment Company, Nonconsolidated Subsidiary [Line Items]
Fair Value	$ 291.0		$ 286.0
Pharmaceuticals, Biotechnology & Life Sciences | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	2.10%		2.00%
Real Estate Management & Development
Investment Company, Nonconsolidated Subsidiary [Line Items]
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]
Fair Value	$ 2,245.0		$ 2,472.0
Software & Services | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	16.10%		16.90%
Technology Hardware & Equipment
Investment Company, Nonconsolidated Subsidiary [Line Items]
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]
Fair Value	$ 68.0		$ 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]
Fair Value	$ 306.0		$ 454.0
Transportation | Percentage  of Portfolio | Industry Concentration Risk
Investment Company, Nonconsolidated Subsidiary [Line Items]
Percentage of Portfolio	2.20%		3.10%

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

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

[3] Security is classified as Level 1 or 2 in the Company's fair value hierarchy (see Note 8).

[4] Security is non-income producing.

[5] Security or portion thereof is pledged as collateral supporting the amounts outstanding under the Senior Secured Revolving Credit Facility (see Note 9).

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

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