Derivative Instruments (Details 4) - Not Designated as Hedging Instrument - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	$ (135)	$ (140)
Unrealized loss	(18)	(56)
Purchases	4	12
Settlements	44	49
Outstanding net asset (liability), Ending Balance	(105)	(135)
NUGs
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	(136)	(151)
Unrealized loss	(15)	(47)
Purchases	0	0
Settlements	44	62
Outstanding net asset (liability), Ending Balance	(107)	(136)
Regulated FTRs
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	1	11
Unrealized loss	(3)	(9)
Purchases	4	12
Settlements	0	(13)
Outstanding net asset (liability), Ending Balance	$ 2	$ 1