Portfolio Investments - Schedule of Composition of Investments (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Sep. 30, 2024
Schedule of Investments [Line Items]
Investment owned, at cost	$ 3,012,589	$ 3,144,919
Investments at fair value	2,809,377	3,021,279
Debt investments, fair value	$ 2,641,253	$ 2,846,410
% of Debt Portfolio	100.00%	100.00%
Investments in debt securities
Schedule of Investments [Line Items]
Investment owned, at cost	$ 2,583,956	$ 2,723,134
Investments at fair value	2,481,533	2,684,858
Investments in equity securities
Schedule of Investments [Line Items]
Investment owned, at cost	208,453	202,670
Investments at fair value	153,243	152,328
Debt investments in the JVs
Schedule of Investments [Line Items]
Investment owned, at cost	165,389	164,324
Investments at fair value	159,720	161,552
Equity investments in the JVs
Schedule of Investments [Line Items]
Investment owned, at cost	54,791	54,791
Investments at fair value	14,881	22,541
Floating rate debt securities, including the debt investments in the JVs
Schedule of Investments [Line Items]
Debt investments, fair value	$ 2,401,581	$ 2,516,316
% of Debt Portfolio	90.93%	88.40%
Fixed rate debt securities
Schedule of Investments [Line Items]
Debt investments, fair value	$ 239,672	$ 330,094
% of Debt Portfolio	9.07%	11.60%