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Derivative Instruments (Details 4) - Not Designated as Hedging Instrument - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2016
Sep. 30, 2015
Sep. 30, 2016
Sep. 30, 2015
Outstanding net asset (liability) [Roll Forward]        
Outstanding net asset (liability), Beginning Balance $ (120) $ (128) $ (135) $ (140)
Unrealized loss (6) (24) (18) (39)
Purchases     4 12
Settlements 12 14 35 29
Outstanding net asset (liability), Ending Balance (114) (138) (114) (138)
NUGs        
Outstanding net asset (liability) [Roll Forward]        
Outstanding net asset (liability), Beginning Balance (124) (140) (136) (151)
Unrealized loss (6) (20) (17) (36)
Purchases     0 0
Settlements 12 17 35 44
Outstanding net asset (liability), Ending Balance (118) (143) (118) (143)
Regulated FTRs        
Outstanding net asset (liability) [Roll Forward]        
Outstanding net asset (liability), Beginning Balance 4 12 1 11
Unrealized loss 0 (4) (1) (3)
Purchases     4 12
Settlements 0 (3) 0 (15)
Outstanding net asset (liability), Ending Balance $ 4 $ 5 $ 4 $ 5