Income Taxes -Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Sep. 30, 2019
Income Tax Disclosure [Abstract]
Effective income tax rate	15.90%	17.80%	1.80%	11.40%
Unrecognized tax benefits, uncertain tax positions	$ 6.8		$ 6.8		$ 5.8
Unrecognized tax benefits, accrued interest	$ 0.4		$ 0.4		$ 0.3