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Asset Retirement Obligations (Tables)
 12 Months Ended
Dec. 31, 2020
Asset Retirement Obligation [Abstract]  
Asset retirement obligations The results of the reviews (identified as Revisions in estimated cash outflows) and other changes in asset retirement obligations during 2020 and 2019, were as follows:
 20202019
(Dollars in millions)  
Balance at beginning of year$342 $307 
Additional liabilities accrued5 13 
Revisions in estimated cash outflows11 2 
Acquisition of assets— 1 
Disposition of assets(1)(1)
Accretion expense20 20 
Balance at end of year$377 $342