Employee Retirement Benefits - Net Periodic Income Cost Regulatory Assets (Details) - Pension Benefits [Member] - USD ($) $ in Millions |
12 Months Ended | |
|---|---|---|
Dec. 31, 2020 |
Dec. 31, 2019 |
|
| Defined Benefit Plan Disclosure [Line Items] | ||
| Defined Benefit Plan, Service Cost Recognized In Regulatory Assets Liabilities | $ 1 | $ 0 |
| Unrecognized gains | (89) | (113) |
| Amortization of prior service cost | 1 | 1 |
| Amortization Of Unrecognized Losses Recognized In Regulatory Assets Liabilities | (12) | 0 |
| Total | $ (99) | $ (112) |