Portfolio Investments - Schedule of Composition of Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2026	Sep. 30, 2025
Schedule of Investments [Line Items]
Investment owned, at cost	$ 2,996,697	$ 3,075,122
Investments at fair value	2,741,814	2,847,782
Debt investments, fair value	$ 2,605,350	$ 2,694,714
% of Debt Portfolio	100.00%	100.00%
Investments in debt securities
Schedule of Investments [Line Items]
Investment owned, at cost	$ 2,589,260	$ 2,646,823
Investments at fair value	2,479,522	2,535,998
Investments in equity securities
Schedule of Investments [Line Items]
Investment owned, at cost	185,702	207,729
Investments at fair value	107,736	141,122
Debt investments in the JVs
Schedule of Investments [Line Items]
Investment owned, at cost	138,778	165,779
Investments at fair value	125,828	158,716
Equity investments in the JVs
Schedule of Investments [Line Items]
Investment owned, at cost	82,957	54,791
Investments at fair value	28,728	11,946
Floating rate debt securities, including the debt investments in the JVs
Schedule of Investments [Line Items]
Debt investments, fair value	$ 2,380,987	$ 2,442,837
% of Debt Portfolio	91.39%	90.65%
Fixed rate debt securities
Schedule of Investments [Line Items]
Debt investments, fair value	$ 224,363	$ 251,877
% of Debt Portfolio	8.61%	9.35%