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Asset Retirement Obligations (Schedule of Reconciliation of the Carrying Amount of AROs) (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Asset Retirement Obligation Disclosure [Abstract]    
Beginning balance $ 489 $ 541
Liabilities settled (4) (29)
Accretion expense 23 23
Revisions in estimated cash flows [1] 89 (46)
Ending balance $ 597 $ 489
[1] The increase in 2020 reflects revisions from the nuclear decommissioning cost study. The decrease in 2019 reflects a change in the estimated timing of cash flows for interim pipeline replacements and DOE recoveries.