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Asset Retirement Obligation (Tables) | 3 Months Ended
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Mar. 31, 2014
Changes to Company's Asset Retirement Obligations | The following table describes the changes to the Company’s asset retirement obligations during the three months ended March 31, 2014 and 2013 (in thousands):
|  | Three Months Ended March 31, |
|  | 2014 |  |  | 2013 |
Balance at beginning of year |  | $ | 4,837 |  |  | $ | 4,422 |
Liabilities incurred |  |  | 91 |  |  |  | 71 |
Liabilities settled |  |  | (11 | ) |  |  | (13 | )
Accretion expense |  |  | 42 |  |  |  | 41 |
Asset retirement obligation at end of period |  | $ | 4,959 |  |  | $ | 4,521 |

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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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