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Asset Retirement Obligations (Details)
$ in Millions
9 Months Ended
Sep. 30, 2021
USD ($)
Asset Retirement Obligation Disclosure [Abstract]  
Balance at beginning of year $ 249
Additional liabilities accrued 8
Revisions in estimated cash outflows 43
Disposition of assets (1)
Accretion expense 11
Balance at end of year $ 310