Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 600,442	$ 357,587	$ 586,093
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent	21.00%		35.00%
Income Tax Disclosure [Abstract]
Income taxes at statutory rates (federal and state)	$ 15,198	26,600
Increase (decrease):
Differences required to be flowed-through by regulatory commissions	849	1,518
Other, net	(585)	(1,195)
Total provision for income taxes	$ 15,462	$ 26,923
Effective tax rate	27.10%	40.00%
Income Taxes Gross-up [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 56,500
Estimated Revenue Deferral Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	6,500
TCJA Revaluation [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	213,300		$ 213,300
Estimated Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 219,795	$ 0	$ 213,306