INVESTMENTS - Schedule of Investments (Details) $ in Millions	6 Months Ended	12 Months Ended
Jun. 30, 2023 USD ($) borrower	Dec. 31, 2022 USD ($) borrower
Schedule of Investments [Line Items]
Amortized Cost	$ 21,685.6	$ 22,043.0
Fair Value	$ 21,496.5	$ 21,779.8
Loans on non-accrual status, as a percent of total investments at amortized cost	2.10%	1.70%
Loans on non-accrual status, as a percent of total investments at fair value	1.10%	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	$ 5,096.0	$ 5,166.0
Fair Value	$ 4,799.0	$ 4,958.0
Number of borrowers \| borrower	21	22
Ivy Hill Asset Management, L.P.
Schedule of Investments [Line Items]
Amortized Cost	$ 2,436.0	$ 2,370.0
Fair Value	$ 9,476.0	$ 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	24.90%	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	10.00%	10.10%
Midwest \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	23.90%	23.90%
Southeast \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	18.30%	17.10%
Northeast \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	14.30%	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	$ 4,846.2	$ 4,950.2
Fair Value	$ 4,721.4	$ 4,760.6
Software and Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	22.00%	21.90%
Financial Services
Schedule of Investments [Line Items]
Amortized Cost	$ 2,615.9	$ 2,755.3
Fair Value	$ 2,794.2	$ 2,890.6
Financial Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	13.00%	13.30%
Health Care Services
Schedule of Investments [Line Items]
Amortized Cost	$ 2,639.8	$ 2,525.9
Fair Value	$ 2,425.2	$ 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	$ 2,168.8	$ 2,116.3
Fair Value	$ 2,090.4	$ 2,077.0
Commercial and Professional Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	9.70%	9.50%
Investment Funds and Vehicles
Schedule of Investments [Line Items]
Amortized Cost	$ 1,310.8	$ 1,305.8
Fair Value	$ 1,275.9	$ 1,296.8
Investment Funds and Vehicles \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	5.90%	6.00%
Power Generation
Schedule of Investments [Line Items]
Amortized Cost	$ 976.6	$ 942.8
Fair Value	$ 1,037.6	$ 988.3
Power Generation \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	4.80%	4.50%
Insurance Services
Schedule of Investments [Line Items]
Amortized Cost	$ 1,027.7	$ 1,152.6
Fair Value	$ 1,024.3	$ 1,132.8
Insurance Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	4.80%	5.20%
Consumer Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	3.70%	4.20%
Consumer Durables and Apparel
Schedule of Investments [Line Items]
Amortized Cost	$ 951.1	$ 923.9
Fair Value	$ 793.1	$ 814.3
Consumer Durables and Apparel \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	3.70%	3.70%
Capital Goods
Schedule of Investments [Line Items]
Amortized Cost	$ 569.5	$ 758.3
Fair Value	$ 617.3	$ 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	$ 478.7	$ 422.3
Fair Value	$ 521.4	$ 452.1
Media and Entertainment \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.40%	2.10%
Food and Beverage
Schedule of Investments [Line Items]
Amortized Cost	$ 436.1	$ 420.4
Fair Value	$ 446.6	$ 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%
Retailing and Distribution
Schedule of Investments [Line Items]
Amortized Cost	$ 431.9	$ 442.3
Fair Value	$ 447.0	$ 475.2
Retailing and Distribution \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.10%	2.20%
Energy
Schedule of Investments [Line Items]
Amortized Cost	$ 394.0	$ 380.6
Fair Value	$ 431.4	$ 425.3
Energy \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.00%	2.00%
Automobiles and Components
Schedule of Investments [Line Items]
Amortized Cost	$ 440.7	$ 473.2
Fair Value	$ 426.2	$ 452.6
Automobiles and Components \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.00%	2.10%
Other \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	7.60%	6.90%
First lien senior secured loans
Schedule of Investments [Line Items]
Amortized Cost	$ 9,304.0	$ 9,684.0
Fair Value	8,972.0	9,373.0
Second lien senior secured loans
Schedule of Investments [Line Items]
Amortized Cost	4,160.0	4,218.0
Fair Value	3,879.0	3,934.0
Subordinated certificates of the SDLP
Schedule of Investments [Line Items]
Amortized Cost	1,289.0	1,274.0
Fair Value	1,237.0	1,249.0
Senior subordinated loans
Schedule of Investments [Line Items]
Amortized Cost	1,147.0	1,163.0
Fair Value	1,054.0	1,079.0
Preferred equity
Schedule of Investments [Line Items]
Amortized Cost	2,161.0	2,095.0
Fair Value	2,140.0	2,027.0
Ivy Hill Asset Management, L.P.
Schedule of Investments [Line Items]
Amortized Cost	1,968.0	2,048.0
Fair Value	2,151.0	2,201.0
Other equity
Schedule of Investments [Line Items]
Amortized Cost	1,657.0	1,561.0
Fair Value	2,063.0	1,917.0
Unitranche loans
Schedule of Investments [Line Items]
Amortized Cost	4,875.0	4,983.0
Fair Value	$ 4,746.0	$ 4,841.0