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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, 2019
Dec. 31, 2018
Asset Retirement Obligation Disclosure [Abstract]    
Beginning balance $ 541 $ 529
Liabilities settled (29) (15)
Accretion expense 23 23
Revisions in estimated cash flows [1] (46) 4
Ending balance $ 489 $ 541
[1] The decrease in 2019 reflects a change in the estimated timing of cash flows for interim pipeline replacements and DOE recoveries.