Employee Retirement Benefits - Unrecognized Amounts (Details) - Pension Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Components of AOCI:
Unrecognized prior service benefit	$ 2	$ 2
Unrecognized losses	(108)	(71)
Total	(106)	(69)
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)	(37)	(27)
Unrecognized amounts included in regulatory assets (liabilities) [Abstract]
Unrecognized prior service benefit	(2)	(3)
Unrecognized losses	263	376
Total	$ 261	$ 373