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ASSET RETIREMENT OBLIGATION: (Tables) | 12 Months Ended
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Dec. 31, 2011
ASSET RETIREMENT OBLIGATION: |
Summary of the asset retirement obligation activity |
|  | 2011 |  | 2010 |
Balance as of January 1 |  | $ | 59.1 |  | $ | 48.9 |
Changes in estimates |  | — |  | 5.3 |
Additions |  | — |  | 1.6 |
Closure payments |  | (0.5 | ) | — |
Accretion expense |  | 3.4 |  | 3.3 |
Balance as of December 31, |  | $ | 62.0 |  | $ | 59.1 |

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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 -Topic 410 -SubTopic 30 -Publisher FASB -Number -Article -Subparagraph -Name Accounting Standards Codification -Paragraph 5 -Section 50 -URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13201-110859 + Details
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