Portfolio Investments - Composition of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022
Schedule of Investments [Line Items]
Investment owned, at cost	$ 3,044,119	$ 2,617,754
Investments at fair value	2,892,420	2,494,111
Debt investments, fair value	$ 2,719,775	$ 2,369,862
% of Debt Portfolio	100.00%	100.00%
Investments in debt securities
Schedule of Investments [Line Items]
Investment owned, at cost	$ 2,654,484	$ 2,294,392
Investments at fair value	2,557,102	2,223,329
Investments in equity securities
Schedule of Investments [Line Items]
Investment owned, at cost	171,858	127,596
Investments at fair value	143,767	103,534
Debt investments in the JVs
Schedule of Investments [Line Items]
Investment owned, at cost	162,986	146,444
Investments at fair value	162,673	146,533
Equity investments in the JVs
Schedule of Investments [Line Items]
Investment owned, at cost	54,791	49,322
Investments at fair value	28,878	20,715
Floating rate debt securities, including the debt investments in the JVs
Schedule of Investments [Line Items]
Debt investments, fair value	$ 2,345,205	$ 2,049,644
% of Debt Portfolio	86.23%	86.49%
Fixed rate debt securities
Schedule of Investments [Line Items]
Debt investments, fair value	$ 374,570	$ 320,218
% of Debt Portfolio	13.77%	13.51%