Leases - Component of Lease Expense (Details) $ in Millions	3 Months Ended	9 Months Ended
	Jun. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)
Leases [Abstract]
Operating lease cost	$ 22.4	$ 76.1
Finance lease cost:
Amortization of ROU assets	0.5	3.7
Interest on lease liabilities	0.7	1.9
Variable lease cost	1.6	4.4
Short-term lease cost	0.8	2.4
Total lease cost	$ 26.0	88.5
Cash paid related to lease liabilities:
Operating cash flows — operating leases		76.3
Operating cash flows — finance leases		1.9
Financing cash flows — finance leases		2.7
Non-cash lease liability activities:
ROU assets obtained in exchange for operating lease liabilities (including the impact upon adoption)		464.3
ROU assets obtained in exchange for finance lease liabilities		$ 21.5
Weighted-average remaining lease term
Operating leases	6 years 2 months 12 days	6 years 2 months 12 days
Finance leases	2 years 3 months 18 days	2 years 3 months 18 days
Weighted-average discount rate
Operating leases	3.90%	3.90%
Finance leases	2.00%	2.00%