Income Taxes - Effective Tax Rate Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Effective Income Tax Rate Reconciliation at Federal Statutory Income Tax Rate, Amount									$ 88.4	$ 97.5	$ 78.0
Effective Income Tax Rate Reconciliation, State and Local Income Taxes, Amount									0.5	2.2	1.3
Effective Income Tax Rate Reconciliation, Foreign Income Tax Rate Differential, Amount									(34.7)	(30.7)	(37.4)
Effective Income Tax Rate Reconciliation, Repatriation of Foreign Earnings, Amount									(0.3)	4.7	(2.0)
Effective Income Tax Rate Reconciliation, Tax Credit, Other, Amount									0.8	0.8	0.7
Effective Income Tax Rate Reconciliation, Change in Deferred Tax Assets Valuation Allowance, Amount									0.9	(19.2)	(6.2)
Effective Income Tax Rate Reconciliation, Other Adjustments, Amount									(1.2)	5.1	(1.3)
Income Tax Expense	$ 11.6	$ 14.4	$ 11.5	$ 15.3	$ 15.6	$ 17.6	$ 12.2	$ 13.4	$ 52.8	$ 58.8	$ 31.7