Leases - Components of Operating Lease Costs Recognized Within Selling, General and Administrative Expenses (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Leases [Abstract]
Operating lease costs	$ 26.4	$ 26.2	$ 79.0	$ 78.9
Variable lease costs	2.2	2.2	6.8	6.6
Total operating lease costs	$ 28.6	$ 28.4	$ 85.8	$ 85.5