Stock-based compensation - Stock Options Outstanding and Activity During the Period (Detail) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended	12 Months Ended
	Jun. 30, 2021	Sep. 30, 2020
Options Outstanding
Balance as of September 30, 2020	2,500,000
Granted	300,000
Exercised	(800,000)
Canceled/Forfeited	(100,000)
Expired	(0.0)
Balance as of June 30, 2021	1,900,000	2,500,000
Vested and expected to vest after June 30, 2021	1,900,000
Exercisable as of June 30, 2021	1,200,000
Weighted-Average Exercise Price
Beginning Balance	$ 33.09
Granted	35.78
Exercised	30.30
Canceled/Forfeited	39.37
Expired	15.47
Ending Balance	34.57	$ 33.09
Vested and expected to vest after June 30, 2021	34.59
Exercisable as of June 30, 2021	$ 35.87
Weighted-Average Remaining Contractual Life
Balance	5 years 10 months 24 days	5 years 10 months 24 days
Vested and expected to vest after June 30, 2021	5 years 10 months 24 days
Exercisable as of June 30, 2021	4 years 7 months 6 days
Aggregate Intrinsic Value
Balance	$ 35.8	$ 6.9
Vested and expected to vest after June 30, 2021	35.1
Exercisable as of June 30, 2021	$ 21.2