Income Taxes - Components of Provision for Income Taxes (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
Current Tax Provision
Federal									$ 44.5	$ 2.4	$ 0.0
State									4.0	0.6	0.6
Foreign									68.9	47.0	58.3
Current Income Tax Provision									117.4	50.0	58.9
Deferred Tax Provision
Federal									0.4	1.3	1.3
State									(0.3)	0.1	0.1
Foreign									1.4	1.6	(1.5)
Deferred Income Tax Provision									1.5	3.0	(0.1)
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