Investments - Investments at Fair Value and Amortized Cost (Details) - USD ($) $ in Thousands	Sep. 30, 2024		Dec. 31, 2023
Schedule of Investments [Line Items]
Amortized Cost	$ 5,053,683	[1],[2]	$ 3,785,797	[3],[4]
Fair Value	5,083,969		3,807,639
First-lien senior secured debt investments
Schedule of Investments [Line Items]
Amortized Cost	4,318,083		3,047,941
Fair Value	4,355,248		3,068,392
Second-lien senior secured debt investments
Schedule of Investments [Line Items]
Amortized Cost	164,425		187,024
Fair Value	156,068		186,796
Unsecured debt investments
Schedule of Investments [Line Items]
Amortized Cost	110,266		72,097
Fair Value	111,413		73,823
Preferred equity investments
Schedule of Investments [Line Items]
Amortized Cost	323,695		374,363
Fair Value	306,747		370,458
Common equity investments
Schedule of Investments [Line Items]
Amortized Cost	135,025		104,372
Fair Value	152,306		108,170
Joint Ventures
Schedule of Investments [Line Items]
Amortized Cost	2,189		0
Fair Value	$ 2,187		$ 0

[1] As of September 30, 2024, the net estimated unrealized gain for U.S. federal income tax purposes was $85.2 million based on a tax cost basis of $5.0 billion. As of September 30, 2024, there was no estimated aggregate gross unrealized loss for U.S. federal income tax purposes and the estimated aggregate gross unrealized gain for U.S. federal income tax purposes was $85.2 million.

[2] The amortized cost represents the original cost adjusted for the amortization and accretion of premiums and discounts, as applicable, on debt investments using the effective interest method.

[3] As of December 31, 2023, the net estimated unrealized gain for U.S. federal income tax purposes was $63.7 million based on a tax cost basis of $3.7 billion. As of December 31, 2023, there was no estimated aggregate gross unrealized loss for U.S. federal income tax purposes and the estimated aggregate gross unrealized gain for U.S. federal income tax purposes was $63.7 million.

[4] The amortized cost represents the original cost adjusted for the amortization and accretion of premiums and discounts, as applicable, on debt investments using the effective interest method.