Portfolio Investments - Schedule of Composition of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Sep. 30, 2024
Schedule of Investments [Line Items]
Investment owned, at cost	$ 3,075,122	$ 3,144,919
Investments at fair value	2,847,782	3,021,279
Debt investments, fair value	$ 2,694,714	$ 2,846,410
% of Debt Portfolio	100.00%	100.00%
Investments in debt securities
Schedule of Investments [Line Items]
Investment owned, at cost	$ 2,646,823	$ 2,723,134
Investments at fair value	2,535,998	2,684,858
Investments in equity securities
Schedule of Investments [Line Items]
Investment owned, at cost	207,729	202,670
Investments at fair value	141,122	152,328
Debt investments in the JVs
Schedule of Investments [Line Items]
Investment owned, at cost	165,779	164,324
Investments at fair value	158,716	161,552
Equity investments in the JVs
Schedule of Investments [Line Items]
Investment owned, at cost	54,791	54,791
Investments at fair value	11,946	22,541
Floating rate debt securities, including the debt investments in the JVs
Schedule of Investments [Line Items]
Debt investments, fair value	$ 2,442,837	$ 2,516,316
% of Debt Portfolio	90.65%	88.40%
Fixed rate debt securities
Schedule of Investments [Line Items]
Debt investments, fair value	$ 251,877	$ 330,094
% of Debt Portfolio	9.35%	11.60%