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Derivative Instruments (Details 4) - Not Designated as Hedging Instrument - USD ($)
$ in Millions
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Outstanding net asset (liability) [Roll Forward]        
Outstanding net asset (liability), Beginning Balance $ (95) $ (120) $ (105) $ (135)
Unrealized loss (4) (6) (15) (18)
Purchases     3 4
Settlements 10 12 28 35
Outstanding net asset (liability), Ending Balance (89) (114) (89) (114)
NUGs        
Outstanding net asset (liability) [Roll Forward]        
Outstanding net asset (liability), Beginning Balance (98) (124) (107) (136)
Unrealized loss (4) (6) (14) (17)
Purchases     0 0
Settlements 10 12 29 35
Outstanding net asset (liability), Ending Balance (92) (118) (92) (118)
Regulated FTRs        
Outstanding net asset (liability) [Roll Forward]        
Outstanding net asset (liability), Beginning Balance 3 4 2 1
Unrealized loss 0 0 (1) (1)
Purchases     3 4
Settlements 0 0 (1) 0
Outstanding net asset (liability), Ending Balance $ 3 $ 4 $ 3 $ 4