Income Taxes - Summary of Reconciliation of Changes in Unrecognized Tax Benefits (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Unrecognized tax benefits, beginning balance	$ 2,323	$ 1,559	$ 1,481
Increases in tax positions for prior years	58	278	61
Decreases in tax positions for prior years	(1)	(2)
Increases in tax positions for current year	664	735	735
Settlements with taxing authorities			(613)
Lapse of the statute of limitations	(173)	(247)	(105)
Unrecognized tax benefits, ending balance	$ 2,871	$ 2,323	$ 1,559