Employee Retirement Benefits - Unrecognized Amounts (Details) - Pension Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Components of AOCI:
Unrecognized prior service benefit (cost) (net of $2 tax expense and $1 tax benefit, respectively)	$ 3	$ (2)
Unrecognized losses (net of $55 and $38 tax benefit, respectively)	(87)	(60)
Total	(84)	(62)
Tax effects on components of AOCI [Abstract]
Tax expense (benefit) related to unrecognized prior service benefit (cost)	2	(1)
Tax expense (benefit) related to unrecognized gain (loss)	(55)	(38)
Unrecognized amounts included in regulatory assets (liabilities) [Abstract]
Unrecognized prior service cost (benefit)	(4)	9
Unrecognized losses	280	232
Total	$ 276	$ 241
Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Expected Long-term Return on Assets	7.75%	7.75%	7.75%