Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2018	Sep. 30, 2017
Income Tax Contingency [Line Items]
U.S. statutory rate			2453.00%
Effective income tax rate	24.10%	17.70%	101.50%	17.20%
Effective income tax rate reconciliation, effect of tax cuts and jobs act of 2017, amount	$ 11.5		$ 491.2
Effective income tax rate reconciliation, effect of tax cuts and jobs act of 2017, percent	3.80%		81.80%
Tax adjustments, settlements, and unusual provisions			$ 94.2
Effective income tax rate reconciliation, repatriation of foreign earnings, amount	$ 11.5		397.0
Tax Cuts and Jobs Act of 2017, incomplete accounting, transition tax for accumulated foreign earnings, provisional liability	397.0		$ 397.0
Transition tax percentage, cash assets			15.50%
Transition tax assets, non-cash assets			8.00%
Deferred tax liabilities, deferred expense	59.8		$ 59.8
Deferred tax assets, deferred income	59.8		59.8
Gross unrecognized tax benefits	28.2		28.2			$ 31.1
Accrued interest and penalties related to unrecognized tax benefits	4.1		4.1			$ 4.0
Reasonably possible amount of reduction in gross unrecognized tax benefits for the next twelve months	7.4		7.4
Reasonably possible amount of net reduction to income tax provision if unrecognized tax benefits were recognized	8.5		$ 8.5
Earliest open year for income tax examinations, U.S. federal			2014
Earliest open tax year for income tax examinations, state, local and non-U.S.			2003
Scenario, Forecast [Member]
Income Tax Contingency [Line Items]
U.S. statutory rate					24.53%
Other Current Liabilities [Member]
Income Tax Contingency [Line Items]
Tax Cuts and Jobs Act of 2017, incomplete accounting, transition tax for accumulated foreign earnings, provisional liability	34.7		$ 34.7
Other Liabilities [Member]
Income Tax Contingency [Line Items]
Tax Cuts and Jobs Act of 2017, incomplete accounting, transition tax for accumulated foreign earnings, provisional liability	$ 362.3		$ 362.3