Income Taxes - Narrative (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2018
Operating Loss Carryforwards [Line Items]
Tax Cuts And Jobs Act of 2017, provisional income tax expense (benefit)		$ (166.0)
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 asset, provisional income tax expense (benefit)		12.6
Tax Cuts And Jobs Act of 2017, incomplete accounting, transition tax for accumulated foreign earnings, provisional income tax expense (benefit)		$ 1.7
Effective income tax rate reconciliation, blended federal rate	(21.00%)	(24.50%)	(24.50%)
Foreign Tax Authority | French Parliament
Operating Loss Carryforwards [Line Items]
Effective income tax rate reconciliation, change in enacted tax rate, amount (excluding tax cuts and jobs act of 2017 Impact)		$ 3.9
Foreign Tax Authority | French Parliament | January 1, 2022 (Fiscal 2023)
Operating Loss Carryforwards [Line Items]
Income tax benefit, continuing operations, adjustment of deferred taxes		$ 17.3