Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended			6 Months Ended	12 Months Ended
	Dec. 31, 2018	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2018	Sep. 30, 2018	Mar. 31, 2019
Operating Loss Carryforwards [Line Items]
Tax Cuts And Jobs Act of 2017, incomplete accounting, change in tax rate, deferred tax liability, provisional income tax (expense) benefit				$ 180.3
Tax Cuts And Jobs Act Of 2017, incomplete accounting, change in tax rate, deferred tax liability, provisional income tax benefit, including unrecognized deferred tax liability		$ 5.0		388.8
Tax Cuts And Jobs Act of 2017, provisional income tax expense (benefit)				(171.3)
Tax Cuts And Jobs Act Of 2017, incomplete accounting, provisional income tax expense (benefit)		(5.3)
Tax Cuts And Jobs Act of 2017, incomplete accounting, change in tax rate, deferred tax asset, provisional income tax expense (benefit)				7.6
Tax Cuts And Jobs Act of 2017, incomplete accounting, transition tax for accumulated foreign earnings, provisional income tax expense (benefit)				1.4
Effective income tax rate reconciliation, blended federal rate	(21.00%)		(24.50%)		(24.50%)
Deferred income taxes		905.9		905.9	$ 991.9	$ 970.8
Foreign Tax Authority | French Parliament | January 1, 2022 (Fiscal 2023)
Operating Loss Carryforwards [Line Items]
Income tax benefit, continuing operations, adjustment of deferred taxes		3.7	$ 17.3
Deferred income taxes		$ 13.6		$ 13.6