INVESTMENTS - Schedule of Investments (Details) $ in Millions	12 Months Ended
	Dec. 31, 2022 USD ($) borrower	Dec. 31, 2021 USD ($) borrower
Schedule of Investments [Line Items]
Amortized Cost	$ 22,043.0	$ 19,810.0
Fair Value	$ 21,779.8	$ 20,009.5
Loans on non-accrual status, as a percent of total investments at amortized cost	1.70%	0.80%
Loans on non-accrual status, as a percent of total investments at fair value	1.10%	0.50%
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	$ 5,166.0	$ 4,193.0
Fair Value	$ 4,958.0	$ 4,127.0
Number of borrowers | borrower	22	19
Ivy Hill Asset Management, L.P.
Schedule of Investments [Line Items]
Amortized Cost	$ 2,370.0	$ 966.0
Fair Value	$ 8,998.0	$ 5,549.0
Number of borrowers | borrower	21
West | Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	24.50%	26.60%
West | Senior Direct Lending Program | Investments, At Fair Value | Customer Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	5.70%	4.90%
West | Ivy Hill Asset Management, L.P. | Investments, At Fair Value | Customer Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	10.10%	4.70%
Midwest | Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	23.90%	27.90%
Southeast | Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	17.10%	17.20%
Mid-Atlantic | Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	14.90%	14.50%
Northeast | Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	14.00%	9.40%
International | Investments, At Fair Value | Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	5.60%	4.40%
Software & Services
Schedule of Investments [Line Items]
Amortized Cost	$ 4,950.2	$ 4,308.2
Fair Value	$ 4,760.6	$ 4,378.1
Software & Services | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	21.90%	21.90%
Diversified Financials
Schedule of Investments [Line Items]
Amortized Cost	$ 2,755.3	$ 1,363.8
Fair Value	$ 2,890.6	$ 1,505.3
Diversified Financials | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	13.30%	7.50%
Health Care Services
Schedule of Investments [Line Items]
Amortized Cost	$ 2,525.9	$ 2,133.9
Fair Value	$ 2,341.9	$ 2,157.1
Health Care Services | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	10.80%	10.80%
Commercial & Professional Services
Schedule of Investments [Line Items]
Amortized Cost	$ 2,116.4	$ 1,820.0
Fair Value	$ 2,077.1	$ 1,833.4
Commercial & Professional Services | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	9.50%	9.20%
Investment Funds and Vehicles
Schedule of Investments [Line Items]
Amortized Cost	$ 1,305.8	$ 1,020.0
Fair Value	$ 1,296.8	$ 1,032.4
Investment Funds and Vehicles | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	6.00%	5.20%
Insurance Services
Schedule of Investments [Line Items]
Amortized Cost	$ 1,152.6	$ 1,144.0
Fair Value	$ 1,132.8	$ 1,167.3
Insurance Services | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	5.20%	5.80%
Power Generation
Schedule of Investments [Line Items]
Amortized Cost	$ 942.8	$ 877.8
Fair Value	$ 988.3	$ 898.9
Power Generation | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	4.50%	4.50%
Consumer Services | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	4.20%	3.90%
Consumer Durables & Apparel
Schedule of Investments [Line Items]
Amortized Cost	$ 923.9	$ 918.7
Fair Value	$ 814.3	$ 874.4
Consumer Durables & Apparel | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	3.70%	4.40%
Capital Goods
Schedule of Investments [Line Items]
Amortized Cost	$ 758.3	$ 945.3
Fair Value	$ 783.9	$ 957.0
Capital Goods | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	3.60%	4.80%
Automobiles & Components
Schedule of Investments [Line Items]
Amortized Cost	$ 574.8	$ 933.1
Fair Value	$ 562.3	$ 910.5
Automobiles & Components | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.60%	4.60%
Media & Entertainment
Schedule of Investments [Line Items]
Amortized Cost	$ 422.3	$ 434.7
Fair Value	$ 452.1	$ 448.2
Media & Entertainment | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.10%	2.20%
Food & Beverage
Schedule of Investments [Line Items]
Amortized Cost	$ 420.4	$ 446.6
Fair Value	$ 431.7	$ 445.3
Food & Beverage | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.00%	2.20%
Energy
Schedule of Investments [Line Items]
Amortized Cost	$ 380.6	$ 363.4
Fair Value	$ 425.3	$ 372.2
Energy | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.00%	1.90%
Pharmaceuticals, Biotechnology & Life Sciences
Schedule of Investments [Line Items]
Amortized Cost	$ 346.9	$ 394.2
Fair Value	$ 367.5	$ 386.8
Pharmaceuticals, Biotechnology & Life Sciences | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	1.70%	1.90%
Other | Investments, At Fair Value | Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	6.90%	9.20%
First lien senior secured loans
Schedule of Investments [Line Items]
Amortized Cost	$ 9,684.0	$ 9,583.0
Fair Value	9,373.0	9,459.0
Second lien senior secured loans
Schedule of Investments [Line Items]
Amortized Cost	4,218.0	4,614.0
Fair Value	3,934.0	4,524.0
Subordinated certificates of the SDLP
Schedule of Investments [Line Items]
Amortized Cost	1,274.0	987.0
Fair Value	1,249.0	987.0
Senior subordinated loans
Schedule of Investments [Line Items]
Amortized Cost	1,163.0	896.0
Fair Value	1,079.0	890.0
Preferred equity
Schedule of Investments [Line Items]
Amortized Cost	2,095.0	1,547.0
Fair Value	2,027.0	1,561.0
Ivy Hill Asset Management, L.P.
Schedule of Investments [Line Items]
Amortized Cost	2,048.0	781.0
Fair Value	2,201.0	936.0
Other equity
Schedule of Investments [Line Items]
Amortized Cost	1,561.0	1,402.0
Fair Value	1,917.0	1,652.0
Unitranche loans
Schedule of Investments [Line Items]
Amortized Cost	4,983.0	5,210.0
Fair Value	$ 4,841.0	$ 5,163.0