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Derivative Instruments and Hedging Activities (Gains and Losses) (Details) - Energy Commodities, net - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2016
Mar. 31, 2015
Cash Flow Hedges    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing) $ 5 $ 7
Cash Flow Hedges | Wholesale energy    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing) (3) (2)
Cash Flow Hedges | Fuel and energy purchases    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing) 8 8
Cash Flow Hedges | Depreciation    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) Recognized in Income on Derivative (Ineffective Portion and Amount Excluded from Effectiveness Testing)   1
Not Designated as Hedging Instrument    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) on derivative, net 205 (75)
Not Designated as Hedging Instrument | Wholesale energy    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) on derivative, net 247 (84)
Not Designated as Hedging Instrument | Retail Energy    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) on derivative, net 19 (39)
Not Designated as Hedging Instrument | Fuel and energy purchases    
Derivative Instruments, Gain (Loss) [Line Items]    
Gain (Loss) on derivative, net $ (61) $ 48