Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income taxes at statutory rates (federal and state)	$ (135)	$ 2,603	$ 15,233	$ 29,912
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Differences required to be flowed-through by regulatory commissions	(14)	66	835	1,584
Other, net	(7)	(122)	(592)	(1,318)
Total provision for income taxes on continuing operations	$ (156)	$ 2,547	$ 15,476	$ 30,178
Effective tax rate for continuing operations	31.50%	38.50%	27.10%	39.90%
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 602,294	$ 359,205	$ 602,294	$ 359,205	$ 586,093
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%		35.00%
Income Taxes Gross-up [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	56,500		$ 56,500		$ 56,500
Estimated Revenue Deferral Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	9,400		9,400
TCJA Revaluation [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	213,300		213,300		213,300
Estimated Regulatory Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory liabilities	$ 222,734	$ 0	$ 222,734	$ 0	$ 213,306