Employee Retirement Benefits - Unrecognized Amounts (Details) - Pension Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017
Components of AOCI:
Unrecognized prior service benefit	$ 2	$ 2
Unrecognized losses	(71)	(49)
Total	(69)	(47)
Tax effects on components of AOCI [Abstract]
Tax expense (benefit) related to unrecognized prior service benefit (cost)	2	2
Tax expense (benefit) related to unrecognized gain (loss)	(27)	(32)
Unrecognized amounts included in regulatory assets (liabilities) [Abstract]
Unrecognized prior service benefit	(3)	(4)
Unrecognized losses	376	160
Total	$ 373	$ 156