Regulatory Assets and Liabilities, Regulatory Liabilities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	$ 16,940	$ 9,717
Regulatory Liability, Noncurrent	132,674	126,424
Plant Removal Costs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	123,105	116,293
Regulatory liability, remaining amortization period	Plant lives
DOE Settlement
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	4,931	6,814
Regulatory Liability, Noncurrent	0	0
Regulatory liability, remaining amortization period	Less than one year
Investment Tax Credit Deferrals
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	0	0
Regulatory Liability, Noncurrent	9,397	9,976
Regulatory liability, remaining amortization period	Various
Conservation Programs
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	1,010	1,187
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	0	1,542
Regulatory Liability, Noncurrent	0	0
Regulatory liability, remaining amortization period	Less than one year
Excess depreciation reserve
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	10,999	0
Regulatory Liability, Noncurrent	0	0
Regulatory liability, remaining amortization period	Various
Other Regulatory Liabilities
Regulatory Liabilities [Line Items]
Regulatory Liability, Current	0	174
Regulatory Liability, Noncurrent	$ 172	$ 155
Regulatory liability, remaining amortization period	Various