INVESTMENTS - Schedule of Investments (Details) $ in Millions	9 Months Ended	12 Months Ended
Sep. 30, 2023 USD ($) borrower	Dec. 31, 2022 USD ($) borrower
Schedule of Investments [Line Items]
Amortized Cost	$ 21,864.0	$ 22,043.0
Fair Value	$ 21,928.5	$ 21,779.8
Loans on non-accrual status, as a percent of total investments at amortized cost	1.20%	1.70%
Loans on non-accrual status, as a percent of total investments at fair value	0.60%	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	$ 4,991.0	$ 5,166.0
Fair Value	$ 4,848.0	$ 4,958.0
Number of borrowers \| borrower	21	22
Ivy Hill Asset Management, L.P.
Schedule of Investments [Line Items]
Amortized Cost	$ 2,314.0	$ 2,370.0
Fair Value	$ 9,491.0	$ 8,998.0
Number of borrowers \| borrower	23
Midwest \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	25.40%	23.90%
West \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	24.40%	24.50%
West \| Senior Direct Lending Program \| Investments, At Fair Value \| Customer Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	5.50%	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	9.20%	10.10%
Southeast \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	17.60%	17.10%
Mid-Atlantic \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	13.60%	14.90%
Northeast \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	13.00%	14.00%
International \| Investments, At Fair Value \| Geographic Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	6.00%	5.60%
Software and Services
Schedule of Investments [Line Items]
Amortized Cost	$ 5,174.8	$ 4,950.2
Fair Value	$ 5,079.5	$ 4,760.6
Software and Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	23.20%	21.90%
Financial Services
Schedule of Investments [Line Items]
Amortized Cost	$ 2,491.7	$ 2,755.3
Fair Value	$ 2,680.3	$ 2,890.6
Financial Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	12.20%	13.30%
Health Care Services
Schedule of Investments [Line Items]
Amortized Cost	$ 2,619.9	$ 2,525.9
Fair Value	$ 2,482.8	$ 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	$ 2,182.5	$ 2,116.3
Fair Value	$ 2,163.3	$ 2,077.0
Commercial and Professional Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	9.90%	9.50%
Investment Funds and Vehicles
Schedule of Investments [Line Items]
Amortized Cost	$ 1,280.6	$ 1,305.8
Fair Value	$ 1,255.4	$ 1,296.8
Investment Funds and Vehicles \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	5.70%	6.00%
Insurance Services
Schedule of Investments [Line Items]
Amortized Cost	$ 1,091.0	$ 1,152.6
Fair Value	$ 1,111.2	$ 1,132.8
Insurance Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	5.10%	5.20%
Power Generation
Schedule of Investments [Line Items]
Amortized Cost	$ 907.8	$ 942.8
Fair Value	$ 967.8	$ 988.3
Power Generation \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	4.40%	4.50%
Consumer Services \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	3.90%	4.20%
Consumer Durables and Apparel
Schedule of Investments [Line Items]
Amortized Cost	$ 905.4	$ 923.9
Fair Value	$ 750.0	$ 814.3
Consumer Durables and Apparel \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	3.40%	3.70%
Media and Entertainment
Schedule of Investments [Line Items]
Amortized Cost	$ 551.8	$ 422.3
Fair Value	$ 602.8	$ 452.1
Media and Entertainment \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.80%	2.10%
Capital Goods
Schedule of Investments [Line Items]
Amortized Cost	$ 458.0	$ 758.3
Fair Value	$ 511.7	$ 783.9
Capital Goods \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.30%	3.60%
Energy
Schedule of Investments [Line Items]
Amortized Cost	$ 456.1	$ 380.6
Fair Value	$ 509.6	$ 425.3
Energy \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.30%	2.00%
Food and Beverage
Schedule of Investments [Line Items]
Amortized Cost	$ 437.0	$ 420.4
Fair Value	$ 453.0	$ 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	$ 434.3	$ 442.3
Fair Value	$ 441.9	$ 475.2
Retailing and Distribution \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	2.00%	2.20%
Consumer Staples Distribution and Retail
Schedule of Investments [Line Items]
Amortized Cost	$ 384.4	$ 320.7
Fair Value	$ 426.5	$ 346.5
Consumer Staples Distribution and Retail \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	1.90%	1.60%
Other \| Investments, At Fair Value \| Industry Concentration Risk
Schedule of Investments [Line Items]
Composition, percent of fair value	7.50%	7.40%
First lien senior secured loans
Schedule of Investments [Line Items]
Amortized Cost	$ 9,689.0	$ 9,684.0
Fair Value	9,450.0	9,373.0
Second lien senior secured loans
Schedule of Investments [Line Items]
Amortized Cost	4,023.0	4,218.0
Fair Value	3,775.0	3,934.0
Subordinated certificates of the SDLP
Schedule of Investments [Line Items]
Amortized Cost	1,259.0	1,274.0
Fair Value	1,216.0	1,249.0
Senior subordinated loans
Schedule of Investments [Line Items]
Amortized Cost	1,057.0	1,163.0
Fair Value	1,006.0	1,079.0
Preferred equity
Schedule of Investments [Line Items]
Amortized Cost	2,345.0	2,095.0
Fair Value	2,347.0	2,027.0
Ivy Hill Asset Management, L.P.
Schedule of Investments [Line Items]
Amortized Cost	1,825.0	2,048.0
Fair Value	2,020.0	2,201.0
Other equity
Schedule of Investments [Line Items]
Amortized Cost	1,666.0	1,561.0
Fair Value	2,115.0	1,917.0
Unitranche loans
Schedule of Investments [Line Items]
Amortized Cost	5,254.0	4,983.0
Fair Value	$ 5,128.0	$ 4,841.0