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Derivative Accounting - Gains and Losses from Derivative Instruments (Details) - Commodity Contracts - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
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 $ 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) (1,298,000) (1,459,000) (3,255,000) (5,232,000)
Not Designated as Hedging Instruments        
Gains and losses from derivative instruments        
Net gain (loss) recognized in income (35,062,000) (50,349,000) (4,621,000) (84,654,000)
Not Designated as Hedging Instruments | Revenue        
Gains and losses from derivative instruments        
Net gain (loss) recognized in income 41,000 560,000 524,000 445,000
Not Designated as Hedging Instruments | Fuel and purchased power        
Gains and losses from derivative instruments        
Net gain (loss) recognized in income (35,103,000) (50,909,000) (5,145,000) (85,099,000)
Other comprehensive income | Designated as Hedging Instruments        
Gains and losses from derivative instruments        
Gain (loss) recognized in OCI on derivative instruments (effective portion) $ (47,000) $ (247,000) $ 13,000 $ (534,000)