Employee Retirement Benefits - Net Periodic Income Cost Regulatory Assets (Details) - Pension Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Prior service benefit	$ (12)	$ 0
Unrecognized losses	48	104
Amortization of prior service benefit	(1)	(1)
Total	$ 35	$ 103