Income Tax (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income taxes at statutory rates (federal and state)	$ (4,136)	$ (5,440)	$ 11,097	$ 24,472
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Differences required to be flowed-through by regulatory commissions	(266)	(302)	569	1,282
Other, net	117	20	(475)	(1,298)
Total provision (benefit) for income taxes on continuing operations	$ (4,285)	$ (5,722)	$ 11,191	$ 24,456
Effective tax rate for continuing operations	27.80%	42.00%	26.80%	39.40%
Regulatory Liabilities [Line Items]
Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate, Percent			21.00%		35.00%
Regulatory Liability, Noncurrent	$ 606,175	$ 363,838	$ 606,175	$ 363,838	$ 586,093
Prepaid Expenses and Other Current Assets [Member]
Regulatory Liabilities [Line Items]
Tax Adjustments, Settlements, and Unusual Provisions	7,300
Other Noncurrent Liabilities [Member]
Regulatory Liabilities [Line Items]
Tax Adjustments, Settlements, and Unusual Provisions	4,000
Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Tax Adjustments, Settlements, and Unusual Provisions	3,300
TCJA Revaluation [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	216,600		216,600		213,300
Income Taxes Gross-up [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	57,400		57,400		$ 56,500
Estimated Revenue Deferral Liability [Member]
Regulatory Liabilities [Line Items]
Regulatory Liability, Noncurrent	$ 7,200		$ 7,200