Derivative Instruments (Details 4) - Not Designated as Hedging Instrument - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	$ (105)	$ (135)
Unrealized loss	(10)	(18)
Purchases	3	4
Settlements	36	44
Outstanding net asset (liability), Ending Balance	(76)	(105)
NUGs
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	(107)	(136)
Unrealized loss	(9)	(15)
Purchases	0	0
Settlements	37	44
Outstanding net asset (liability), Ending Balance	(79)	(107)
Regulated FTRs
Outstanding net asset (liability) [Roll Forward]
Outstanding net asset (liability), Beginning Balance	2	1
Unrealized loss	(1)	(3)
Purchases	3	4
Settlements	(1)	0
Outstanding net asset (liability), Ending Balance	$ 3	$ 2