Leases - Components of Lease Expense (Details) $ in Millions	3 Months Ended	6 Months Ended
	Mar. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)
Leases [Abstract]
Operating lease cost	$ 28.1	$ 53.7
Finance lease cost:
Amortization of ROU assets	1.6	3.2
Interest on lease liabilities	0.7	1.2
Variable lease cost	1.4	2.8
Short-term lease cost	0.6	1.6
Total lease cost	$ 32.4	62.5
Cash paid related to lease liabilities:
Operating cash flows from operating leases		53.3
Operating cash flows from finance leases		1.2
Financing cash flows from finance leases		1.9
Non-cash lease liability activities:
ROU assets obtained in exchange for operating lease liabilities (including the impact upon adoption)		475.6
ROU assets obtained in exchange for finance lease liabilities		$ 21.5
Weighted-average remaining lease term
Operating leases	6 years 3 months 18 days	6 years 3 months 18 days
Finance leases	2 years 4 months 24 days	2 years 4 months 24 days
Weighted-average discount rate
Operating leases	3.90%	3.90%
Finance leases	2.00%	2.00%