Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 17,428	$ 11,781
Regulatory Liability, Noncurrent	$ 148,189	141,289
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Plant lives
Regulatory Liability, Current	$ 0	0
Regulatory Liability, Noncurrent	$ 139,735	132,311
Investment Tax Credit Deferrals
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Various
Regulatory Liability, Current	$ 0	0
Regulatory Liability, Noncurrent	$ 8,342	8,869
Deferred Electric Production And Natural Gas Costs
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year
Regulatory Liability, Current	$ 11,377	9,386
Regulatory Liability, Noncurrent	$ 0	0
DOE Settlement
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year
Regulatory Liability, Current	$ 4,822	1,996
Regulatory Liability, Noncurrent	$ 0	0
Conservation Programs
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year
Regulatory Liability, Current	$ 1,122	339
Regulatory Liability, Noncurrent	$ 0	0
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Various
Regulatory Liability, Current	$ 107	60
Regulatory Liability, Noncurrent	$ 112	$ 109