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Asset Retirement Obligations (Tables)
12 Months Ended
Dec. 31, 2021
Asset Retirement Obligation [Abstract]  
Asset retirement obligations The results of the reviews and other changes in asset retirement obligations during 2021 and 2020, were as follows:
 20212020
(Dollars in millions)  
Balance at beginning of year$377 $342 
Additional liabilities accrued28 
Revisions in estimated cash outflows42 11 
Disposition of assets(1)(1)
Accretion expense23 20 
Balance at end of year$469 $377