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ASSET RETIREMENT OBLIGATION: (Tables) | 6 Months Ended
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Jun. 30, 2021
ASSET RETIREMENT OBLIGATION: |
Summary of asset retirement obligation activity | ​ The following table summarizes the asset retirement obligation activity for the first six months of 2021 and 2020 (in millions):
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2021 |  | 2020
Balance as of January 1 | ​ | $ | 545.0 | ​ | $ | 262.3
Closure payments | ​ |  | (1.9) | ​ |  | (0.8)
Accretion expense | ​ |  | 12.2 | ​ |  | 7.4
Balance as of June 30, | ​ | $ | 555.3 | ​ | $ | 268.9

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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://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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