Income Taxes - Effective Tax Rate Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Effective Income Tax Rate Reconciliation, Percent [Abstract]
U.S. federal tax									$ 79.4	$ 88.4	$ 97.5
State taxes, net of federal benefit									0.8	0.5	2.2
Foreign tax differentials									(3.6)	(34.7)	(30.7)
U.S. taxes on foreign earnings									(0.9)	(0.3)	4.7
Other credit and incentives									1.7	0.8	0.8
U.S. tax law change									41.7	0.0	0.0
Valuation allowance									0.6	0.9	(19.2)
Other									2.6	(1.0)	5.1
Income Tax Expense	$ 30.1	$ 19.6	$ 14.2	$ 55.0	$ 11.9	$ 14.4	$ 11.5	$ 15.2	$ 118.9	$ 53.0	$ 58.8