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Asset Retirement Obligations (Tables) | 12 Months Ended
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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:
| 2021 |  | 2020
(Dollars in millions) |  |  |
Balance at beginning of year | $ | 377 |  |  | $ | 342 |
Additional liabilities accrued | 28 |  |  | 5 |
Revisions in estimated cash outflows | 42 |  |  | 11 |
Disposition of assets | (1) |  |  | (1) |
Accretion expense | 23 |  |  | 20 |
Balance at end of year | $ | 469 |  |  | $ | 377 |

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- Definition Tabular disclosure of the changes in carrying amount of a liability for asset retirement obligations, for changes such as new obligations, changes in estimates of existing obligations, spending on existing obligations, property dispositions, and foreign currency translation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 30 -Section 50 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13201-110859 + Details
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