Employee Retirement Benefits - Unrecognized Amounts (Details) - Pension Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Components of AOCI:
Unrecognized prior service benefit (net of $2 and $2 tax expense, respectively)		$ 2	$ 3
Unrecognized losses (net of $32 and $55 tax benefit, respectively)		(49)	(87)
Total		(47)	(84)
Tax effects on components of AOCI [Abstract]
Tax expense (benefit) related to unrecognized prior service benefit (cost)		2	2
Tax expense (benefit) related to unrecognized gain (loss)		(32)	(55)
Unrecognized amounts included in regulatory assets (liabilities) [Abstract]
Unrecognized prior service benefit		(4)	(4)
Unrecognized losses		160	280
Total		$ 156	$ 276
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Rate of Return on Plan Assets	7.35%