Income Taxes - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	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	$ 5.0		$ 388.8
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	34.0		54.5
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)	1.1			$ 5.0
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	$ 13.6
Increase in per share, basic and diluted resulting from future income tax reduction (in dollars per share)		$ 0.08
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%