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ASSET RETIREMENT OBLIGATION: (Tables) | 6 Months Ended
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Jun. 30, 2019
Summary of asset retirement obligation activity | ​ The following table summarizes the asset retirement obligation activity for the six months ended June 30, 2019 and 2018 (in millions): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2019 |  | 2018
Balance as of January 1, | ​ | $ | 217.7 | ​ | $ | 222.5
Changes in estimates | ​ |  | 25.2 | ​ |  | (15.6)
Closure payments | ​ |  | (0.4) | ​ |  | —
Accretion expense | ​ |  | 6.2 | ​ |  | 6.2
Balance as of June 30, | ​ | $ | 248.7 | ​ | $ | 213.1
​

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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://fasb.org/us-gaap/role/ref/legacyRef -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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