Employee Retirement Benefits - Net Periodic Income Cost Regulatory Assets (Details) - Pension Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Prior service benefit	$ 0	$ (12)
Unrecognized losses (gains)	(120)	48
Amortization of prior service cost (benefit)	1	(1)
Total	$ (119)	$ 35