Income Taxes (Details) - USD ($) $ in Thousands		3 Months Ended	6 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Mar. 31, 2018	Sep. 30, 2018
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate				24.50%
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