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ASSET RETIREMENT OBLIGATION: (Tables) | 9 Months Ended
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Sep. 30, 2016
ASSET RETIREMENT OBLIGATION: |
Summary of asset retirement obligation activity | The following table summarizes the asset retirement obligation activity for the nine months ended September 30, 2016 and 2015 ($ in millions):
|  | 2016 |  | 2015 |
Balance as of January 1 |  | $ | 190.9 |  | $ | 116.1 |
Changes in estimates |  | — |  | — |
Payments |  | (1.7 | ) | (15.3 | )
Accretion expense |  | 21.2 |  | 13.1 |
Balance as of September 30, |  | $ | 210.4 |  | $ | 113.9 |

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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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