Regulatory Assets and Liabilities, Regulatory Liabilities (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 11,781	$ 16,940
Regulatory Liability, Noncurrent	141,289	132,674
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	$ 132,311	123,105
Regulatory liability, remaining amortization period	Plant lives
DOE Settlement
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 1,996	4,931
Regulatory Liability, Noncurrent	$ 0	0
Regulatory liability, remaining amortization period	One to two years
Investment Tax Credit Deferrals
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 0	0
Regulatory Liability, Noncurrent	$ 8,869	9,397
Regulatory liability, remaining amortization period	Various
Conservation Programs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 339	1,010
Regulatory Liability, Noncurrent	$ 0	0
Regulatory liability, remaining amortization period	Less than one year
Deferred Electric Production And Natural Gas Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 9,386	0
Regulatory Liability, Noncurrent	$ 0	0
Regulatory liability, remaining amortization period	Less than one year
Excess depreciation reserve
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 60	10,999
Regulatory Liability, Noncurrent	$ 0	0
Regulatory liability, remaining amortization period	Less than one year
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 0	0
Regulatory Liability, Noncurrent	$ 109	$ 172
Regulatory liability, remaining amortization period	Various