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ASSET RETIREMENT OBLIGATION: (Tables) | 6 Months Ended
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Jun. 30, 2013
ASSET RETIREMENT OBLIGATION: |
Summary of the asset retirement obligation activity | The following table summarizes the asset retirement obligation activity for the six months ended June 30, 2013 and 2012 (in millions):
|  | 2013 |  | 2012 |
Balance as of January 1 |  | $ | 118.2 |  | $ | 62.0 |
Closure payments |  | (0.7 | ) | — |
Accretion expense |  | 4.9 |  | 1.7 |
Balance as of June 30, |  | $ | 122.4 |  | $ | 63.7 |

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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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