Investment Portfolio - Schedule Of Investment Portfolio At Cost And Fair Value (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2023		Dec. 31, 2022
Schedule of Investments [Line Items]
Amortized Cost	$ 16,182.6		$ 16,314.1
Fair Value	15,277.1		15,377.3	[1]
Net Senior Secured Loans—First Lien
Schedule of Investments [Line Items]
Amortized Cost	9,635.4		9,607.5
Fair Value	9,320.4	[2]	9,278.0
Net Senior Secured Loans—Second Lien
Schedule of Investments [Line Items]
Amortized Cost	1,279.6		1,299.2
Fair Value	1,172.1	[2]	1,194.5	[1]
Other Senior Secured Debt
Schedule of Investments [Line Items]
Amortized Cost	152.0		152.0
Fair Value	107.0		110.0
Net Subordinated Debt
Schedule of Investments [Line Items]
Amortized Cost	399.0		383.9
Fair Value	280.5	[2]	265.0	[1]
Net Asset Based Finance
Schedule of Investments [Line Items]
Amortized Cost	1,916.2		2,023.6
Fair Value	1,778.5	[2]	1,902.5	[1]
Credit Opportunities Partners JV, LLC
Schedule of Investments [Line Items]
Amortized Cost	1,571.7		1,571.7
Fair Value	1,396.6	[2]	1,428.3	[1]
Equity/Other
Schedule of Investments [Line Items]
Amortized Cost	1,229.0		1,276.0
Fair Value	$ 1,222.0		$ 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	61.00%		60.30%
Percentage  of Portfolio | Customer Concentration Risk | Net Senior Secured Loans—Second Lien
Schedule of Investments [Line Items]
Percentage of Portfolio	7.70%		7.80%
Percentage  of Portfolio | Customer Concentration Risk | Other Senior Secured Debt
Schedule of Investments [Line Items]
Percentage of Portfolio	0.70%		0.70%
Percentage  of Portfolio | Customer Concentration Risk | Net Subordinated Debt
Schedule of Investments [Line Items]
Percentage of Portfolio	1.80%		1.70%
Percentage  of Portfolio | Customer Concentration Risk | Net Asset Based Finance
Schedule of Investments [Line Items]
Percentage of Portfolio	11.70%		12.40%
Percentage  of Portfolio | Customer Concentration Risk | Credit Opportunities Partners JV, LLC
Schedule of Investments [Line Items]
Percentage of Portfolio	9.10%		9.30%
Percentage  of Portfolio | Customer Concentration Risk | Equity/Other
Schedule of Investments [Line Items]
Percentage of Portfolio	8.00%		7.80%

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

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