Investment Portfolio - Investment Portfolio At Cost And Fair Value (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2024		Dec. 31, 2023
Schedule of Investments [Line Items]
Amortized Cost	$ 14,805.6		$ 15,420.0
Fair Value	14,220.8	[1]	14,649.0
Senior Secured Loans—First Lien
Schedule of Investments [Line Items]
Amortized Cost	8,372.3		8,890.1	[2],[3]
Fair Value	8,110.7	[1]	8,529.1	[2],[3],[4]
Senior Secured Loans—Second Lien
Schedule of Investments [Line Items]
Amortized Cost	982.9		1,158.1
Fair Value	1,003.8	[1]	1,089.8	[4]
Other Senior Secured Debt
Schedule of Investments [Line Items]
Amortized Cost	159.0		106.0
Fair Value	154.0		99.0
Subordinated Debt
Schedule of Investments [Line Items]
Amortized Cost	326.5		308.3
Fair Value	334.2	[1]	322.6	[4]
Asset Based Finance
Schedule of Investments [Line Items]
Amortized Cost	2,220.4		2,237.1
Fair Value	2,078.8	[1]	2,077.3	[4]
Credit Opportunities Partners JV, LLC
Schedule of Investments [Line Items]
Amortized Cost	1,571.7		1,571.7
Fair Value	1,390.3	[1]	1,396.9	[4]
Equity/Other
Schedule of Investments [Line Items]
Amortized Cost	1,173.0		1,149.0
Fair Value	$ 1,149.0		$ 1,134.0
Percentage  of Portfolio | Customer Concentration Risk
Schedule of Investments [Line Items]
Percentage of Portfolio	100.00%		100.00%
Percentage  of Portfolio | Customer Concentration Risk | Senior Secured Loans—First Lien
Schedule of Investments [Line Items]
Percentage of Portfolio	57.00%		58.20%
Percentage  of Portfolio | Customer Concentration Risk | Senior Secured Loans—Second Lien
Schedule of Investments [Line Items]
Percentage of Portfolio	7.10%		7.50%
Percentage  of Portfolio | Customer Concentration Risk | Other Senior Secured Debt
Schedule of Investments [Line Items]
Percentage of Portfolio	1.10%		0.70%
Percentage  of Portfolio | Customer Concentration Risk | Subordinated Debt
Schedule of Investments [Line Items]
Percentage of Portfolio	2.30%		2.20%
Percentage  of Portfolio | Customer Concentration Risk | Asset Based Finance
Schedule of Investments [Line Items]
Percentage of Portfolio	14.60%		14.20%
Percentage  of Portfolio | Customer Concentration Risk | Credit Opportunities Partners JV, LLC
Schedule of Investments [Line Items]
Percentage of Portfolio	9.80%		9.50%
Percentage  of Portfolio | Customer Concentration Risk | Equity/Other
Schedule of Investments [Line Items]
Percentage of Portfolio	8.10%		7.70%

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

[2] The investment is not a qualifying asset under the Investment Company Act of 1940, as amended. A business development company may not acquire any asset other than qualifying assets, unless, at the time the acquisition is made, qualifying assets represent at least 70% of the company’s total assets. As of December 31, 2023, 73.1% of the Company’s total assets represented qualifying assets.

[3] Security or portion thereof is pledged as collateral supporting the amounts outstanding under the Senior Secured Revolving Credit Facility (see Note 9).

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