Income Taxes (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Dec. 31, 2017	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
Increase in regulatory liabilities, excess deferred income taxes	$ 87,803
Scenario, Forecast
Operating Loss Carryforwards [Line Items]
Federal statutory income tax rate		24.50%