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ASSET RETIREMENT OBLIGATION: (Tables) | 12 Months Ended
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Dec. 31, 2015
ASSET RETIREMENT OBLIGATION: |
Summary of asset retirement obligation activity | The following table summarizes the asset retirement obligation activity for years ended December 31, 2015 and 2014 (in millions):
|  | 2015 |  | 2014 |
Balance as of January 1 |  | $ | 116.1 |  | $ | 124.8 |
Changes in estimates |  | 3.8 |  | (9.6 | )
Additions |  | 72.2 |  | — |
Closure payments |  | (14.6 | ) | (12.2 | )
Accretion expense |  | 13.4 |  | 13.1 |
Balance as of December 31, |  | $ | 190.9 |  | $ | 116.1 |

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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 -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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