Investment Portfolio - Schedule of Investment Portfolio At Cost And Fair Value (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Jun. 30, 2023		Dec. 31, 2022
Schedule of Investments [Line Items]
Amortized Cost	$ 15,542.8		$ 16,314.1
Fair Value	14,764.0	[1]	15,377.3	[2]
Net Senior Secured Loans—First Lien
Schedule of Investments [Line Items]
Amortized Cost	9,127.9		9,607.5
Fair Value	8,781.8	[1]	9,278.0
Net Senior Secured Loans—Second Lien
Schedule of Investments [Line Items]
Amortized Cost	1,263.2		1,299.2
Fair Value	1,170.4	[1]	1,194.5	[2]
Other Senior Secured Debt
Schedule of Investments [Line Items]
Amortized Cost	173.0		152.0
Fair Value	129.0		110.0
Subordinated Debt
Schedule of Investments [Line Items]
Amortized Cost	276.6		383.9
Fair Value	288.1	[1]	265.0	[2]
Asset Based Finance
Schedule of Investments [Line Items]
Amortized Cost	1,938.9		2,023.6
Fair Value	1,779.5	[1]	1,902.5	[2]
Credit Opportunities Partners JV, LLC
Schedule of Investments [Line Items]
Amortized Cost	1,571.7		1,571.7
Fair Value	1,380.8	[1]	1,428.3	[2]
Equity/Other
Schedule of Investments [Line Items]
Amortized Cost	1,191.0		1,276.0
Fair Value	$ 1,235.0		$ 1,199.0
Percentage  of Portfolio | Customer Concentration Risk
Schedule of Investments [Line Items]
Percentage of Portfolio	100.00%		100.00%
Percentage  of Portfolio | Customer Concentration Risk | Net Senior Secured Loans—First Lien
Schedule of Investments [Line Items]
Percentage of Portfolio	59.50%		60.30%
Percentage  of Portfolio | Customer Concentration Risk | Net Senior Secured Loans—Second Lien
Schedule of Investments [Line Items]
Percentage of Portfolio	7.90%		7.80%
Percentage  of Portfolio | Customer Concentration Risk | Other Senior Secured Debt
Schedule of Investments [Line Items]
Percentage of Portfolio	0.90%		0.70%
Percentage  of Portfolio | Customer Concentration Risk | Subordinated Debt
Schedule of Investments [Line Items]
Percentage of Portfolio	1.90%		1.70%
Percentage  of Portfolio | Customer Concentration Risk | Asset Based Finance
Schedule of Investments [Line Items]
Percentage of Portfolio	12.00%		12.40%
Percentage  of Portfolio | Customer Concentration Risk | Credit Opportunities Partners JV, LLC
Schedule of Investments [Line Items]
Percentage of Portfolio	9.40%		9.30%
Percentage  of Portfolio | Customer Concentration Risk | Equity/Other
Schedule of Investments [Line Items]
Percentage of Portfolio	8.40%		7.80%

[1] See Note 8 for additional information regarding the fair value of the Company's financial instruments

[2] Fair value determined by the Company’s board of directors (see Note 8).