Stock-based compensation - Activity Under The Equity Incentive Plans (Detail) $ / shares in Units, shares in Thousands, $ in Thousands	6 Months Ended	12 Months Ended
	Mar. 31, 2022 USD ($) $ / shares shares	Sep. 30, 2021 USD ($) $ / shares shares
Shares available
Additional shares authorized (in shares)	249
2018 Equity Incentive Plan
Shares available
Beginning balance (in shares)	1,728
Additional shares authorized (in shares)	1,000
Stock options granted (in shares)	(214)
Stock options forfeited (in shares)	89
Restricted stock units granted (in shares)	(1,150)
Forfeiture of restricted stock units (in shares)	67
Shares withheld for payment of taxes (in shares)	62
Ending balance (in shares)	1,582	1,728
Options outstanding
Beginning balance (in shares)	3,132
Stock options granted (in shares)	214
Stock options exercised (in shares)	(368)
Stock options forfeited (in shares)	(89)
Ending balance (in shares)	2,889	3,132
Vested or expected to vest (in shares)	2,889
Vested and exercisable (in shares)	1,739
Weighted average exercise price per share
Beginning balance (in usd per share) | $ / shares	$ 27.15
Stock options granted (in usd per share) | $ / shares	26.14
Stock options exercised (in usd per share) | $ / shares	13.05
Stock options forfeited (in usd per share) | $ / shares	31.96
Ending balance (in usd per share) | $ / shares	28.79	$ 27.15
Vested or expected to vest (in usd per share) | $ / shares	28.79
Vested and exercisable (in usd per share) | $ / shares	$ 21.00
Weighted average remaining contractual term (years)
Weighted average remaining contractual term, Outstanding	7 years	7 years 3 months 18 days
Weighted average remaining contractual term, Vested or expected to vest	7 years
Weighted average remaining contractual term, Vested and exercisable	6 years 4 months 24 days
Aggregate intrinsic value
Aggregate intrinsic value, Outstanding | $	$ 70,722	$ 251,343
Aggregate intrinsic value, Vested or expected to vest | $	70,722
Aggregate intrinsic value, Vested and exercisable | $	$ 51,266