Employee Retirement Benefits - Net Periodic Income Cost Regulatory Assets (Details) - Pension Benefits [Member] - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Prior service benefit	$ 0	$ (12)
Unrecognized losses	104	226
Amortization of prior service benefit	(1)	(3)
Total	$ 103	$ 211