Income Taxes -Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Sep. 30, 2020
Income Tax Disclosure [Abstract]
Effective income tax rate	1.70%	(31.10%)
Unrecognized tax benefits, uncertain tax positions	$ 9.0		$ 8.0
Unrecognized tax benefits, accrued interest	$ 0.5		$ 0.4