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Derivative Instruments (Details 4) - Not Designated as Hedging Instrument - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Outstanding net asset (liability) [Roll Forward]        
Outstanding net asset (liability), Beginning Balance $ (105) $ (137) $ (105) $ (135)
Unrealized loss (5)   (11) (12)
Purchases 3 4 3 4
Settlements 12 13 18 23
Outstanding net asset (liability), Ending Balance (95) (120) (95) (120)
NUGs        
Outstanding net asset (liability) [Roll Forward]        
Outstanding net asset (liability), Beginning Balance (103) (135) (107) (136)
Unrealized loss (5)   (10) (12)
Purchases 0 0 0 0
Settlements 10 11 19 24
Outstanding net asset (liability), Ending Balance (98) (124) (98) (124)
Regulated FTRs        
Outstanding net asset (liability) [Roll Forward]        
Outstanding net asset (liability), Beginning Balance (2) (2) 2 1
Unrealized loss 0   (1) 0
Purchases 3 4 3 4
Settlements 2 2 (1) (1)
Outstanding net asset (liability), Ending Balance $ 3 $ 4 $ 3 $ 4