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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)