Leases - Lease Expenses (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021		Sep. 30, 2020
Lease, Cost [Abstract]
Operating lease cost	$ 141.4	[1]	$ 72.5
Variable lease cost	21.2		6.0
Finance lease cost	0.4		0.5
Sublease income	(24.6)		(4.2)
Total lease expense	$ 138.4		$ 74.8

[1] Substantially all operating lease cost is included in occupancy expense.