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ASSET RETIREMENT OBLIGATION: (Tables) | 3 Months Ended
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Mar. 31, 2017
ASSET RETIREMENT OBLIGATION: |
Summary of asset retirement obligation activity | The following table summarizes the asset retirement obligation activity for the three months ended March 31, 2017 and 2016 (in millions):
|  | 2017 |  | 2016 |
Balance as of January 1 |  | $ | 216.5 |  | $ | 190.9 |
Changes in estimates |  | — |  | — |
Additions |  | — |  | — |
Payments |  | (0.3 | ) | (0.7 | )
Accretion expense |  | 6.1 |  | 6.8 |
Balance as of March 31, |  | $ | 222.3 |  | $ | 197.0 |

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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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