Income Taxes - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2018
Operating Loss Carryforwards [Line Items]
Reduction in net deferred income tax liabilities due to the remeasuring of existing federal deferred income tax assets and liabilities	$ 383.8
Discrete deferred income tax adjustments	$ 166.0
Change In Income Tax Rate, Effect of Change on Earnings Per Share, Basic	$ 0.96
Change In Income Tax Rate, Effect of Change on Earnings Per Share, Diluted	$ 0.94
Reduction in deferred income tax liabilities related to tax reform	$ 180.3
Establishment of valuation allowances related to deferred tax assets due to tax reform	12.6
Toll tax on un-repatraited foreign earnings	1.7
Regulatory liability, excess deferred federal income taxes due to tax reform	216.1
Increase in regulatory liabilities for the tax effect of future revenue reductions	87.8
Estimated deferred tax benefit resulting from future income tax reduction	20.4
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
Increase in per share, basic and diluted resulting from future income tax reduction (in dollars per share)		$ 0.10
Foreign Tax Authority | French Parliament | January 1, 2020 (Fiscal 2021)
Operating Loss Carryforwards [Line Items]
Income tax benefit, continuing operations, adjustment of deferred taxes	$ 27.4
Increase in per share, basic and diluted resulting from future income tax reduction (in dollars per share)		$ 0.15
Scenario, Forecast
Operating Loss Carryforwards [Line Items]
Effective income tax rate reconciliation, blended federal rate			24.50%