Leases - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Lessee Lease Description [Line Items]
Weighted-average remaining lease term - operating leases	7 years 1 month 6 days
Weighted-average discount rate - operating leases	4.90%
Minimum initial or remaining lease term	1 year
Rent expense for the fiscal years		$ 26.2	$ 23.3
Minimum
Lessee Lease Description [Line Items]
Operating lease expiration year	2020
Operating lease, term of contract	1 year
Maximum
Lessee Lease Description [Line Items]
Operating lease expiration year	2033
Operating lease, term of contract	15 years
Operating lease, option to extend, term	10 years