Fair Value of Financial Instruments - Schedule of Investments at Amortized Cost and Fair Value (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2024		Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 3,514,013	[1]	$ 769,035	[2]
Fair Value	$ 3,476,614	[1]	$ 756,145	[2]
Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	100.00%		100.00%
Senior Secured First Lien Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 2,633,399		$ 642,976
Fair Value	$ 2,603,522		$ 632,343
Senior Secured First Lien Debt | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	74.90%		83.60%
Senior Secured Second Lien Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 164,109		$ 55,145
Fair Value	$ 161,058		$ 52,126
Senior Secured Second Lien Debt | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	4.60%		6.90%
Subordinated Debt
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 137,258		$ 35,389
Fair Value	$ 137,147		$ 35,500
Subordinated Debt | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	3.90%		4.70%
Collateralized Securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 20,046
Fair Value	$ 17,409
Collateralized Securities | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	0.50%
Equity/Other
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 253,767		$ 35,525
Fair Value	$ 252,544		$ 36,176
Equity/Other | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	7.30%		4.80%
FBLC Senior Loan Fund, LLC
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amortized Cost	$ 305,434
Fair Value	$ 304,934
FBLC Senior Loan Fund, LLC | Investment Owned, At Fair Value | Investment Type Concentration Risk
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair Value Percentage of Total Portfolio	8.80%

[1] Percentages are based on net assets attributable to common stock as of March 31, 2024.

[2] Percentages are based on net assets attributable to common stock as of December 31, 2023.