Income Taxes - Reconciliation of Beginning and Ending Amount of Unrecognized Tax Benefits (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Reconciliation of Unrecognized Tax Benefits, Excluding Amounts Pertaining to Examined Tax Returns [Roll Forward]
Gross unrecognized tax benefits at beginning of year	$ 5,834	$ 6,113	$ 6,480
Gross increases for tax positions in prior years	883	509	404
Gross decreases for tax positions in prior years	(65)	(611)	0
Gross increases based on tax positions related to the current year	2,260	1,439	1,625
Decreases for settlements and payments	0	(637)	0
Decreases due to statute expiration	(918)	(979)	(2,396)
Gross unrecognized tax benefits at end of year	$ 7,994	$ 5,834	$ 6,113