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Derivative Accounting - Gains and Losses from Derivative Instruments (Details) - Commodity Contracts - USD ($)
3 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Designated as Hedging Instruments    
Gains and losses from derivative instruments    
Amount reclassified from accumulated other comprehensive income to earnings related to discontinued cash flow hedges $ 0 $ 0
Designated as Hedging Instruments | Fuel and purchased power    
Gains and losses from derivative instruments    
Loss Reclassified from Accumulated OCI into Income (Effective Portion Realized) (491,000) (851,000)
Not Designated as Hedging Instruments    
Gains and losses from derivative instruments    
Net loss recognized in income (35,308,000) (52,915,000)
Not Designated as Hedging Instruments | Operating revenues    
Gains and losses from derivative instruments    
Net loss recognized in income (1,219,000) (288,000)
Not Designated as Hedging Instruments | Fuel and purchased power    
Gains and losses from derivative instruments    
Net loss recognized in income (34,089,000) (52,627,000)
Other comprehensive income | Designated as Hedging Instruments    
Gains and losses from derivative instruments    
Loss Recognized in OCI on Derivative Instruments (Effective Portion) $ 0 $ (96,000)