Change in Net Assets from Operations Per Common Share - Summary of Information Sets Forth for Computation of Basic and Diluted Per Share Net Increase in Net Assets Resulting from Operations (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Numerator for net increase (decrease) in net assets resulting from operations	$ (24,740)	$ 166,617	$ (15,970)
Denominator for basic and diluted weighted average shares	66,254,150	67,045,105	67,045,105
Basic and diluted net increase (decrease) in net assets per share resulting from operations	$ (0.37)	$ 2.49	$ (0.24)