Stock option plan - Schedule of Nonvested Restricted Stock Units Activity (Detail) - Restricted Stock Units (RSUs) [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
Nonvested Restricted Stock Units Activity [Line Items]
Number of shares, Outstanding Beginning Balance	462,000
Number of shares, granted	312,000
Number of shares, vested	(175,000)
Number of shares, forfeited	(30,000)
Number of shares, Outstanding Ending Balance	569,000	462,000
Expected to vest, Outstanding	569,000
Weighted average grant date fair value price per share, Outstanding Beginning Balance	$ 26.16
Weighted average grant date fair value price per share, granted	38.66
Weighted average grant date fair value price per share, vested	26.36
Weighted average grant date fair value price per share, forfeited	26.12
Weighted average grant date fair value price per share, Outstanding Ending Balance	32.96	$ 26.16
Weighted average grant date fair value price per share, Expected to vest	$ 32.96
Weighted average remaining contractual term, granted	3 years 2 months 12 days	3 years 10 months 24 days
Weighted average remaining contractual term, Expected to vest	3 years 2 months 12 days
Aggregate intrinsic value, Outstanding	$ 43,260	$ 8,959
Aggregate intrinsic value, Expected to vest	$ 43,260