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Asset Retirement Obligations (Tables) | 12 Months Ended
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Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract] |
Schedule of Changes to the Company's Before-tax Asset Retirement Obligation | The following table indicates the changes to the company’s before-tax asset retirement obligations in 2017, 2016 and 2015:
| 2017 |  |  |  | 2016 |  |  | 2015 |
Balance at January 1 | $ | 14,243 |  |  |  | $ | 15,642 |  |  | $ | 15,053 |
Liabilities incurred | 684 |  |  |  | 204 |  |  | 51 |
Liabilities settled | (1,721 | ) |  |  | (1,658 | ) |  | (981 | )
Accretion expense | 668 |  |  |  | 749 |  |  | 715 |
Revisions in estimated cash flows | 340 |  |  |  | (694 | ) |  | 804 |
Balance at December 31 | $ | 14,214 |  |  |  | $ | 14,243 |  |  | $ | 15,642 |

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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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