Income taxes - Summary of Income Tax Contingencies (Details) - USD ($) $ in Thousands	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Reconciliation of the beginning and ending amount of gross unrecognized tax benefits
Balance at beginning of year	$ 721	$ 633	$ 448
Increases for tax positions taken in current year	2,280	88	185
Balance at end of year	$ 3,001	$ 721	$ 633