Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Income Tax Disclosure [Abstract]
Effective income tax rate	33.50%	179.40%
U.S. statutory rate		24.53%
Effective income tax rate reconciliation, effect of tax cuts and jobs act of 2017, amount		$ 479.7
Effective income tax rate reconciliation, effect of tax cuts and jobs act of 2017, percent		161.10%
Provisional expense related to revaluation of U.S. deferred tax assets and liabilities		$ 104.4
Provisional expense related to transition tax liability for foreign subsidiaries		$ 395.8
Gross unrecognized tax benefits	$ 20.1		$ 20.1
Accrued interest and penalties related to unrecognized tax benefits	2.7		$ 2.5
Reasonably possible amount of reduction in gross unrecognized tax benefits for the next twelve months	6.0
Reasonably possible amount of net reduction to income tax provision if unrecognized tax benefits were recognized	$ 5.6