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Derivative Accounting - Gains and Losses from Derivative Instruments (Details) - Commodity Contracts - USD ($)
12 Months Ended
Dec. 31, 2015
Dec. 31, 2014
Dec. 31, 2013
Designated as Hedging Instruments      
Derivative Instruments in Designated Cash Flows Hedges      
Loss Recognized in OCI on Derivative Instruments (Effective Portion) $ (615,000) $ (372,000) $ (353,000)
Gain (Loss) Reclassified from Accumulated OCI into Income (Effective Portion Realized) (5,988,000) (21,415,000) (44,219,000)
Gain Recognized in Income (Ineffective Portion and Amount Excluded from Effectiveness Testing) 0 0 0
Amount reclassified from accumulated other comprehensive income to earnings related to discontinued cash flow hedges 0 0 0
Not Designated as Hedging Instruments      
Derivative Instruments Not Designated as Cash Flows Hedges      
Net Gain (Loss) Recognized in Income (108,399,000) (66,043,000) (10,160,000)
Revenue | Not Designated as Hedging Instruments      
Derivative Instruments Not Designated as Cash Flows Hedges      
Net Gain (Loss) Recognized in Income 574,000 324,000 289,000
Fuel and purchased power | Not Designated as Hedging Instruments      
Derivative Instruments Not Designated as Cash Flows Hedges      
Net Gain (Loss) Recognized in Income $ (108,973,000) $ (66,367,000) $ (10,449,000)