Stock option plan - Activity Under The Equity Incentive Plans (Detail) - 2018 Equity Incentive Plan [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares available, Outstanding Beginning Balance	1,521,000
Shares available, Additional shares authorized	1,000,000
Shares available, Stock options granted	(1,481,000)
Shares available, Stock options forfeited	203,000
Shares available, Restricted stock units granted	(312,000)
Shares available, Early exercised options repurchased	2,000
Shares available, Forfeiture of restricted common stock	30,000
Shares available, Shares withheld for payment of taxes	71,000
Shares available, Outstanding Ending Balance	1,034,000	1,521,000
Number of options, Outstanding beginning balance	3,550,000
Options outstanding, Stock options granted	1,481,000
Options outstanding, Stock options exercised	(915,000)
Options outstanding, Stock options forfeited	(203,000)
Number of options, Outstanding ending balance	3,913,000	3,550,000
Options outstanding, Vested or expected to vest	3,913,000
Options outstanding, Vested and exercisable	1,312,000
Weighted average exercise price per share, Outstanding Beginning Balance	$ 15.99
Weighted average exercise price per share, Stock options granted	35.49
Weighted average exercise price per share, Stock options exercised	11.50
Weighted average exercise price per share, Stock options forfeited	23.39
Weighted average exercise price per share, Outstanding Ending Balance	$ 24.35	$ 15.99
Weighted average remaining contractual term, Outstanding	8 years 1 month 6 days	8 years 3 months 18 days
Aggregate intrinsic value, Outstanding Vested or expected to vest and exercisable	$ 204,365	$ 31,998
Weighted average exercise price per share, Vested or expected to vest	$ 24.35
Weighted average remaining contractual term, Vested or expected to vest	8 years 1 month 6 days
Aggregate intrinsic value, Vested or expected to vest	$ 204,365
Weighted average exercise price per share, Vested and exercisable	$ 14.26
Weighted average remaining contractual term,Vested and exercisable	7 years
Aggregate intrinsic value, Vested and exercisable	$ 80,995