Employee Retirement Benefits - Unrecognized Amounts (Details) - Pension Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Components of AOCI:
Unrecognized prior service benefit	$ 2	$ 2
Unrecognized losses	(60)	(108)
Total	(58)	(106)
Tax effects on components of AOCI [Abstract]
Tax expense (benefit) related to unrecognized prior service benefit (cost)	1	2
Tax expense (benefit) related to unrecognized gain (loss)	(24)	(37)
Unrecognized amounts included in regulatory assets (liabilities) [Abstract]
Unrecognized prior service benefit	(1)	(2)
Unrecognized losses	163	263
Total	$ 162	$ 261