Income Taxes (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended		12 Months Ended
	Oct. 01, 2018	Mar. 31, 2018	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Sep. 30, 2017
Income Taxes [Line Items]
Effective Tax Rate		29.40%	37.20%	(17.40%)	38.00%
Federal Statutory Rate		24.50%		24.50%	35.00%	35.00%
Reduction to Income Tax Expense Due to Remeasurement of Deferred Income Tax Assets and Liabilities		$ 4.0		$ 107.0
Reduction In Deferred Taxes for Rate Regulated Activities Due to Remeasurement of Deferred Income Tax Assets and Liabilities				336.7
Decrease to Recoverable Future Taxes Due to Change in Corporate Tax Rate				65.7
Increase in Taxes Refundable to Customers Due to Change in Corporate Tax Rate				271.0
Alternative Minimum Tax Credit [Member]
Income Taxes [Line Items]
Tax Credit Carryforwards		$ 89.6		$ 89.6
Subsequent Event [Member]
Income Taxes [Line Items]
Federal Statutory Rate	21.00%