Regulatory Assets and Liabilities, Regulatory Liabilities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
Regulatory Liabilities [Line Items]
Regulatory liability, current	$ 6,086	$ 16,609
Regulatory liabilities	123,746	119,187
Plant Removal Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Plant lives
Regulatory liability, current	0	0
Regulatory liabilities	113,949	108,850
DOE Settlement [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year
Regulatory liability, current	5,628	14,484
Regulatory liabilities	0	0
Investment Tax Credit Deferrals [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Various
Regulatory liability, current	0	0
Regulatory liabilities	9,626	9,525
Conservation Programs [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year
Regulatory liability, current	73	0
Regulatory liabilities	0	0
Deferred Electric Energy Costs [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Less than one year
Regulatory liability, current	134	1,869
Regulatory liabilities	0	0
Other Regulatory Liabilities [Member]
Regulatory Liabilities [Line Items]
Regulatory liability, remaining amortization period	Various
Regulatory liability, current	251	256
Regulatory liabilities	$ 171	$ 812