Income taxes - Schedule Of Unrecognized Tax Benefits Roll Forward (Detail) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Balance	$ 3,291	$ 2,249	$ 1,625
Increases related to tax positions during the year	1,409	1,042	624
Balance	$ 4,700	$ 3,291	$ 2,249