Leases - Components of Operating Lease Costs Recognized in Condensed Consolidated Statements of Operations Amounts Include Both Continuing and Discontinued Operations (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Leases [Abstract]
Operating lease costs	$ 27.3	$ 25.7
Amortization of right-of-use assets	2.0	1.0
Interest on lease obligations	0.3	0.1
Variable lease costs	2.1	2.3
Total lease costs	$ 31.7	$ 29.1