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Forward Contracts Classified as Derivatives - Effect of marking to market forward contracts for purchase and sale of electricity and location and fair value amounts of related derivatives (Details) (USD $)
In Thousands, unless otherwise specified
Jun. 30, 2013
Dec. 31, 2012
Jun. 30, 2012
Dec. 31, 2011
Derivatives, Fair Value [Line Items]        
Regulatory Asset - Current $ 20,313 $ 25,499    
Regulatory Assets - Long-Term 131,545 134,755    
Current Liability - Marked-to-Market Loss (13,294) (18,234)    
Regulatory Liability - Current 515 559    
Regulatory Liability - Long-Term 70,048 68,835    
Forward Electricity Contracts
       
Derivatives, Fair Value [Line Items]        
Current Asset - Marked-to-Market Gain 1,180 502    
Total Assets 13,789 18,501    
Current Liability - Marked-to-Market Loss (13,294) (18,234)    
Total Liabilities (13,749) (18,452)    
Net Fair Value of Marked-to-Market Energy Contracts 40 49 131 894
Forward Electricity Contracts | Deferred Marked-to-Market Losses
       
Derivatives, Fair Value [Line Items]        
Regulatory Asset - Current 5,572 7,949    
Regulatory Assets - Long-Term 7,037 10,050    
Forward Electricity Contracts | Deferred Marked-to-Market Gain
       
Derivatives, Fair Value [Line Items]        
Regulatory Liability - Current (19) (8)    
Regulatory Liability - Long-Term $ (436) $ (210)