Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended	12 Months Ended
	Mar. 31, 2018	Mar. 31, 2018	Sep. 30, 2018
Operating Loss Carryforwards [Line Items]
Tax Cuts and Jobs Act of 2017, reduction of net deferred income tax liabilities	$ 223,660
Tax Cuts and Jobs Act of 2017, reduction of income tax expense	8,122
Tax Cuts and Jobs Act of 2017, change in tax rate, provisional unrecognized deferred tax liability	216,098	$ 216,098
Increase in regulatory liabilities, excess deferred income taxes		87,803
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount	$ 15,200	$ 23,300
Scenario, Forecast
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate			24.50%