Income Taxes - Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Reconciliation of Unrecognized Tax Benefits
Beginning balance	$ 3,085	$ 1,435	$ 3,091
Increase for tax positions taken during a prior period	2,135	1,401	0
Increase for tax positions taken during the current period	0	249	0
Decrease for tax positions as a result of the lapse of the statute of limitations	(457)	0	(1,656)
Ending balance	$ 4,763	$ 3,085	$ 1,435