INVESTMENTS - Investments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2024 USD ($) borrower		Dec. 31, 2023 USD ($) borrower
Schedule of Investments [Line Items]
Amortized Cost	$ 26,374.0	[1]	$ 22,667.6	[2]
Fair Value	$ 26,719.9	[1]	$ 22,873.7	[2]
Loans on non-accrual status, as a percent of total investments at amortized cost	1.70%		1.30%
Loans on non-accrual status, as a percent of total investments at fair value	1.00%		0.60%
Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	100.00%		100.00%
Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	100.00%		100.00%
Senior Direct Lending Program
Schedule of Investments [Line Items]
Amortized Cost	$ 4,591.0		$ 5,267.0
Fair Value	$ 4,390.0		$ 5,129.0
Number of borrowers | borrower	20		22
Ivy Hill Asset Management, L.P.
Schedule of Investments [Line Items]
Amortized Cost	$ 2,237.0		$ 2,288.0
Fair Value	$ 8,173.0		$ 9,401.0
Number of borrowers | borrower	20
West | Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	25.20%		24.30%
West | Senior Direct Lending Program | Investments, At Fair Value | Customer Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	4.50%		5.60%
West | Ivy Hill Asset Management, L.P. | Investments, At Fair Value | Customer Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	7.10%		8.70%
Midwest | Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	20.90%		24.70%
Southeast | Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	19.30%		18.50%
Mid-Atlantic | Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	16.10%		13.30%
Northeast | Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	12.20%		13.20%
International | Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	6.30%		6.00%
Software and Services
Schedule of Investments [Line Items]
Amortized Cost	$ 6,561.9		$ 5,362.2
Fair Value	$ 6,544.6		$ 5,293.6
Software and Services | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	24.50%		23.20%
Health Care Equipment and Services
Schedule of Investments [Line Items]
Amortized Cost	$ 3,333.6		$ 2,914.8
Fair Value	$ 3,211.9		$ 2,768.9
Health Care Equipment and Services | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	12.00%		12.10%
Financial Services
Schedule of Investments [Line Items]
Amortized Cost	$ 2,400.2		$ 2,391.6
Fair Value	$ 2,644.8		$ 2,617.8
Financial Services | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	9.90%		11.40%
Commercial and Professional Services
Schedule of Investments [Line Items]
Amortized Cost	$ 2,486.8		$ 2,317.0
Fair Value	$ 2,510.0		$ 2,329.5
Commercial and Professional Services | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	9.40%		10.20%
Insurance
Schedule of Investments [Line Items]
Amortized Cost	$ 1,510.1		$ 1,113.4
Fair Value	$ 1,573.5		$ 1,176.0
Insurance | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	5.90%		5.10%
Consumer Distribution and Retail | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	5.70%		4.60%
Consumer Services | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	5.10%		4.10%
Investment Funds And Vehicles
Schedule of Investments [Line Items]
Amortized Cost	$ 1,288.2		$ 1,338.3
Fair Value	$ 1,238.4		$ 1,329.2
Investment Funds And Vehicles | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	4.60%		5.80%
Media and Entertainment
Schedule of Investments [Line Items]
Amortized Cost	$ 1,214.2		$ 584.7
Fair Value	$ 1,218.3		$ 649.7
Media and Entertainment | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	4.60%		2.80%
Independent Power and Renewable Electricity Producers
Schedule of Investments [Line Items]
Amortized Cost			$ 804.0
Fair Value			$ 880.9
Independent Power and Renewable Electricity Producers | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	3.70%		3.80%
Capital Goods
Schedule of Investments [Line Items]
Amortized Cost	$ 710.9		$ 496.6
Fair Value	$ 736.5		$ 549.4
Capital Goods | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.70%		2.40%
Pharmaceuticals, Biotechnology and Life Sciences
Schedule of Investments [Line Items]
Amortized Cost	$ 705.8		$ 334.1
Fair Value	$ 695.4		$ 353.8
Pharmaceuticals, Biotechnology and Life Sciences | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.60%		1.50%
Consumer Durables and Apparel
Schedule of Investments [Line Items]
Amortized Cost	$ 644.7		$ 877.9
Fair Value	$ 606.4		$ 718.6
Consumer Durables and Apparel | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.30%		3.10%
Food and Beverage
Schedule of Investments [Line Items]
Amortized Cost	$ 479.4
Fair Value	$ 479.6
Food and Beverage | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	1.80%		2.10%
Automobiles and Components
Schedule of Investments [Line Items]
Amortized Cost	$ 392.9		$ 393.9
Fair Value	$ 383.8		$ 381.5
Automobiles and Components | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	1.40%		1.70%
Other | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	3.80%		6.10%
First lien senior secured loans
Schedule of Investments [Line Items]
Amortized Cost	$ 15,519.0		$ 10,313.0
Fair Value	15,179.0		10,081.0
Second lien senior secured loans
Schedule of Investments [Line Items]
Amortized Cost	1,935.0		3,980.0
Fair Value	1,847.0		3,753.0
Subordinated certificates of the SDLP
Schedule of Investments [Line Items]
Amortized Cost	1,263.0		1,316.0
Fair Value	1,192.0		1,288.0
Senior subordinated loans
Schedule of Investments [Line Items]
Amortized Cost	1,384.0		1,150.0
Fair Value	1,351.0		1,096.0
Preferred equity
Schedule of Investments [Line Items]
Amortized Cost	2,667.0		2,457.0
Fair Value	2,649.0		2,460.0
Ivy Hill Asset Management, L.P.
Schedule of Investments [Line Items]
Amortized Cost	1,701.0		1,763.0
Fair Value	1,915.0		1,987.0
Other equity
Schedule of Investments [Line Items]
Amortized Cost	1,905.0		1,689.0
Fair Value	2,587.0		2,209.0
Unitranche loans
Schedule of Investments [Line Items]
Amortized Cost	8,772.0		5,786.0
Fair Value	$ 8,624.0		$ 5,667.0

[1] As of December 31, 2024, the estimated net unrealized gain for federal tax purposes was $0.2 billion based on a tax cost basis of $26.5 billion. As of December 31, 2024, the estimated aggregate gross unrealized gain for federal income tax purposes was $1.7 billion and the estimated aggregate gross unrealized loss for federal income tax purposes was $1.5 billion.     

[2] As of December 31, 2023, the estimated net unrealized loss for federal tax purposes was $0.4 billion based on a tax cost basis of $22.5 billion. As of December 31, 2023, the estimated aggregate gross unrealized loss for federal income tax purposes was $1.7 billion and the estimated aggregate gross unrealized gain for federal income tax purposes was $1.3 billion.