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Asset Retirement Obligations (Schedule of Reconciliation of the Carrying Amount of AROs) (Detail)
$ in Millions
9 Months Ended
Sep. 30, 2020
USD ($)
Asset Retirement Obligation Disclosure [Abstract]  
Beginning balance $ 489
Liabilities settled (4)
Accretion expense 17
Revisions in estimated cash flows 89 [1]
Ending balance $ 591
[1] The increase in 2020 reflects revisions from the nuclear decommissioning cost study.