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ASSET RETIREMENT OBLIGATION: (Tables) | 9 Months Ended
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Sep. 30, 2021
ASSET RETIREMENT OBLIGATION: |
Summary of asset retirement obligation activity | ​ The following table summarizes the asset retirement obligation activity for the first nine months of 2021 and 2020 (in millions):
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2021 |  | 2020
Balance as of January 1 | ​ | $ | 545.0 | ​ | $ | 262.3
Closure payments | ​ |  | (3.7) | ​ |  | (1.0)
Accretion expense | ​ |  | 18.3 | ​ |  | 11.1
Balance as of September 30, | ​ | $ | 559.6 | ​ | $ | 272.4

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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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