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ASSET RETIREMENT OBLIGATION: (Tables) | 12 Months Ended
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Dec. 31, 2020
ASSET RETIREMENT OBLIGATION: |
Summary of asset retirement obligation activity | The following table summarizes the asset retirement obligation activity for the years ended December 31, 2020 and 2019 (in millions):
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2020 |  | 2019
Balance as of January 1 | ​ | $ | 262.3 | ​ | $ | 217.7
Changes in estimates | ​ |  | 269.3 | ​ |  | 36.4
Closure payments | ​ |  | (1.2) | ​ |  | (5.3)
Accretion expense | ​ |  | 14.6 | ​ |  | 13.5
Balance as of December 31, | ​ | $ | 545.0 | ​ | $ | 262.3

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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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