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Derivative Instruments (Details 4) - Not Designated as Hedging Instrument - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2016
Jun. 30, 2015
Jun. 30, 2016
Jun. 30, 2015
Outstanding net asset (liability) [Roll Forward]        
Outstanding net asset (liability), Beginning Balance $ (137) $ (147) $ (135) $ (140)
Unrealized gain (loss)   (8) (12) (15)
Purchases 4 12 4 12
Settlements 13 15 23 15
Outstanding net asset (liability), Ending Balance (120) (128) (120) (128)
NUGs        
Outstanding net asset (liability) [Roll Forward]        
Outstanding net asset (liability), Beginning Balance (135) (148) (136) (151)
Unrealized gain (loss)   (8) (12) (16)
Purchases 0 0 0 0
Settlements 11 16 24 27
Outstanding net asset (liability), Ending Balance (124) (140) (124) (140)
Regulated FTRs        
Outstanding net asset (liability) [Roll Forward]        
Outstanding net asset (liability), Beginning Balance (2) 1 1 11
Unrealized gain (loss)   0 0 1
Purchases 4 12 4 12
Settlements 2 (1) (1) (12)
Outstanding net asset (liability), Ending Balance $ 4 $ 12 $ 4 $ 12