INVESTMENTS - Schedule of Investments (Details) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2023 USD ($) borrower	Dec. 31, 2022 USD ($) borrower
Schedule of Investments [Line Items]
Amortized Cost	$ 21,429.6	$ 22,043.0
Fair Value	$ 21,148.0	$ 21,779.8
Loans on non-accrual status, as a percent of total investments at amortized cost	2.30%	1.70%
Loans on non-accrual status, as a percent of total investments at fair value	1.30%	1.10%
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	$ 5,210.0	$ 5,166.0
Fair Value	$ 4,932.0	$ 4,958.0
Number of borrowers \| borrower	22	22
Ivy Hill Asset Management, L.P.
Schedule of Investments [Line Items]
Amortized Cost	$ 2,444.0	$ 2,370.0
Fair Value	$ 9,549.0	$ 8,998.0
Number of borrowers \| borrower	22
West \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	25.30%	24.50%
West \| Senior Direct Lending Program \| Investments, At Fair Value \| Customer Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	5.80%	5.70%
West \| Ivy Hill Asset Management, L.P. \| Investments, At Fair Value \| Customer Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	10.90%	10.10%
Midwest \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	23.80%	23.90%
Southeast \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	17.30%	17.10%
Northeast \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	15.00%	14.00%
Mid-Atlantic \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	13.80%	14.90%
International \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	4.80%	5.60%
Software and Services
Schedule of Investments [Line Items]
Amortized Cost	$ 4,700.8	$ 4,950.2
Fair Value	$ 4,557.7	$ 4,760.6
Software and Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	21.60%	21.90%
Financial Services
Schedule of Investments [Line Items]
Amortized Cost	$ 2,792.5	$ 2,755.3
Fair Value	$ 2,929.7	$ 2,890.6
Financial Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	13.80%	13.30%
Health Care Services
Schedule of Investments [Line Items]
Amortized Cost	$ 2,623.2	$ 2,525.9
Fair Value	$ 2,382.6	$ 2,341.9
Health Care Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	11.30%	10.80%
Commercial and Professional Services
Schedule of Investments [Line Items]
Amortized Cost	$ 2,132.0	$ 2,116.3
Fair Value	$ 2,067.8	$ 2,077.0
Commercial and Professional Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	9.80%	9.50%
Investment Funds and Vehicles
Schedule of Investments [Line Items]
Amortized Cost	$ 1,313.4	$ 1,305.8
Fair Value	$ 1,280.4	$ 1,296.8
Investment Funds and Vehicles \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	6.10%	6.00%
Insurance Services
Schedule of Investments [Line Items]
Amortized Cost	$ 1,059.3	$ 1,152.6
Fair Value	$ 1,042.1	$ 1,132.8
Insurance Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	4.90%	5.20%
Power Generation
Schedule of Investments [Line Items]
Amortized Cost	$ 957.4	$ 942.8
Fair Value	$ 1,002.6	$ 988.3
Power Generation \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	4.70%	4.50%
Consumer Durables and Apparel
Schedule of Investments [Line Items]
Amortized Cost	$ 946.1	$ 923.9
Fair Value	$ 799.3	$ 814.3
Consumer Durables and Apparel \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	3.80%	3.70%
Consumer Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	3.30%	4.20%
Capital Goods
Schedule of Investments [Line Items]
Amortized Cost	$ 577.0	$ 758.3
Fair Value	$ 609.3	$ 783.9
Capital Goods \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.90%	3.60%
Media and Entertainment
Schedule of Investments [Line Items]
Amortized Cost	$ 428.1	$ 422.3
Fair Value	$ 472.1	$ 452.1
Media and Entertainment \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.20%	2.10%
Retailing and Distribution
Schedule of Investments [Line Items]
Amortized Cost	$ 440.3	$ 442.3
Fair Value	$ 466.4	$ 475.2
Retailing and Distribution \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.20%	2.20%
Automobiles and Components
Schedule of Investments [Line Items]
Amortized Cost	$ 473.4	$ 473.2
Fair Value	$ 459.5	$ 452.6
Automobiles and Components \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.20%	2.10%
Food and Beverage
Schedule of Investments [Line Items]
Amortized Cost	$ 431.8	$ 420.4
Fair Value	$ 443.8	$ 431.7
Food and Beverage \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.10%	2.00%
Energy
Schedule of Investments [Line Items]
Amortized Cost	$ 393.7	$ 380.6
Fair Value	$ 430.7	$ 425.3
Energy \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.00%	2.00%
Other \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	7.10%	6.90%
First lien senior secured loans
Schedule of Investments [Line Items]
Amortized Cost	$ 9,020.0	$ 9,684.0
Fair Value	8,689.0	9,373.0
Second lien senior secured loans
Schedule of Investments [Line Items]
Amortized Cost	4,230.0	4,218.0
Fair Value	3,887.0	3,934.0
Subordinated certificates of the SDLP
Schedule of Investments [Line Items]
Amortized Cost	1,282.0	1,274.0
Fair Value	1,231.0	1,249.0
Senior subordinated loans
Schedule of Investments [Line Items]
Amortized Cost	1,136.0	1,163.0
Fair Value	1,048.0	1,079.0
Preferred equity
Schedule of Investments [Line Items]
Amortized Cost	2,045.0	2,095.0
Fair Value	2,069.0	2,027.0
Ivy Hill Asset Management, L.P.
Schedule of Investments [Line Items]
Amortized Cost	2,139.0	2,048.0
Fair Value	2,295.0	2,201.0
Other equity
Schedule of Investments [Line Items]
Amortized Cost	1,578.0	1,561.0
Fair Value	1,929.0	1,917.0
Unitranche loans
Schedule of Investments [Line Items]
Amortized Cost	4,721.0	4,983.0
Fair Value	$ 4,575.0	$ 4,841.0