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Asset Retirement Obligations (Tables) | 12 Months Ended
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Dec. 31, 2023
Asset Retirement Obligation [Abstract] |
Asset retirement obligations | The results of the reviews and other changes in asset retirement obligations during 2023 and 2022, were as follows:
| 2023 |  | 2022
(Dollars in millions) |  |  |
Balance at beginning of year | $ | 524 |  |  | $ | 469 |
Additional liabilities accrued | 10 |  |  | 19 |
Revisions in estimated cash outflows | (3) |  |  | 11 |
Disposition of assets | (4) |  |  | (1) |
Accretion expense | 29 |  |  | 26 |
Balance at end of year | $ | 556 |  |  | $ | 524 |

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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://www.xbrl.org/2009/role/commonPracticeRef -Topic 410 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147481850/410-20-50-1 + Details
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