Income taxes - Schedule Of Deferred Tax Assets And Liabilities (Detail) - USD ($) $ in Thousands	Sep. 30, 2020	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Net operating loss carryforwards	$ 112,434	$ 73,341
Research and development credit carryforwards	9,407	6,831
Operating lease liability	7,506
Other	5,639	4,553
Gross deferred tax assets	134,986	84,725
Less: Valuation allowance	(127,336)	(84,598)
Net deferred tax assets	7,650	127
Fixed assets	(105)	(1)
Operating lease right-of-use asset	(7,498)
Intangible assets	(47)	(126)
Gross deferred tax liabilities	(7,650)	(127)
Total net deferred tax asset