Stock-based compensation - Restricted Stock Units Activity (Detail) - Restricted Stock Units (RSUs) $ / 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
Number of Shares
Beginning balance (in shares) | shares	698
Granted (in shares) | shares	807
Vested (in shares) | shares	(164)
Forfeited (in shares) | shares	(66)
Ending balance (in shares) | shares	1,275	698
Expected to vest (in shares) | shares	1,275
Weighted average grant date fair value per share
Beginning balance (in usd per share) | $ / shares	$ 73.27
Granted (in usd per share) | $ / shares	79.26
Vested (in usd per share) | $ / shares	72.93
Forfeited (in usd per share) | $ / shares	73.95
Ending balance (in usd per share) | $ / shares	85.41	$ 73.27
Expected to vest (in usd per share) | $ / shares	$ 85.41
Weighted average remaining contractual term (years)
Weighted average remaining contractual term, Outstanding	3 years 2 months 12 days	2 years 8 months 12 days
Weighted average remaining contractual term, Expected to vest	3 years 2 months 12 days
Aggregate Intrinsic Value
Aggregate intrinsic value, Outstanding | $	$ 62,971	$ 74,613
Aggregate intrinsic value, Expected to vest | $	$ 62,971