Summary of Significant Accounting Policies - Components of Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Sep. 30, 2022
Accounting Policies [Abstract]
Foreign currency translation adjustments	$ (28,275)	$ (33,895)
Net unrealized gains (losses) on post-retirement benefits	528	594
Net unrealized losses on available-for-sale securities	(219)	(436)
Accumulated other comprehensive (loss) income	$ (27,966)	$ (33,737)