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Asset retirement obligations ('ARO') (Tables) | 12 Months Ended
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Dec. 31, 2015
Asset Retirement Obligation Disclosure [Abstract] |
Schedule of changes in asset retirement obligations | The reconciliation of the changes in the asset retirement obligations is presented below:
| Year ended December 31,
| 2015 | 2014
Beginning balance, ARO liability | $ | 36,950 |  | $ | 27,113 |
Additional ARO liability incurred | 3,797 |  | 2,548 |
ARO liabilities settled | (5,514 | ) | (4,731 | )
Accretion expense | 1,818 |  | 1,517 |
Adjustments | (1,886 | ) | — |
ARO liability from Mt. Holly acquisition | — |  | 10,503 |
Ending balance, ARO liability | $ | 35,165 |  | $ | 36,950 |

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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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