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Asset Retirement Obligations (Tables) | 12 Months Ended
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Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract] |
Change in asset retirement obligations | The following table shows the change in our asset retirement obligations for 2017 and 2016 (dollars in thousands):
| 2017 |  | 2016
Asset retirement obligations at the beginning of year | $ | 624,475 |  |  | $ | 443,576 |
Changes attributable to: |  |  |  |  |
Accretion expense | 33,104 |  |  | 26,656 |
Settlements | — |  |  | (15,732 | )
Estimated cash flow revisions | 21,950 |  |  | 151,046 |
Newly incurred or acquired obligations | — |  |  | 18,929 |
Asset retirement obligations at the end of year | $ | 679,529 |  |  | $ | 624,475 |

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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/2003/role/presentationRef -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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