Employee Retirement Benefits - Unrecognized Amounts (Details) - Pension Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Components of AOCI:
Unrecognized prior service cost (net of $1 and $1 tax benefit, respectively)	$ (2)	$ (2)
Unrecognized losses (net of $38 and $10 tax benefit, respectively)	(60)	(16)
Total	(62)	(18)
Tax effects on components of AOCI [Abstract]
Tax expense (benefit) related to unrecognized prior service benefit (cost)	(1)	(1)
Tax expense (benefit) related to unrecognized gain (loss)	(38)	(10)
Unrecognized amounts included in regulatory assets (liabilities) [Abstract]
Unrecognized prior service cost	9	10
Unrecognized losses	232	128
Total	$ 241	$ 138