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ASSET RETIREMENT OBLIGATION: (Tables) | 12 Months Ended
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Dec. 31, 2022
ASSET RETIREMENT OBLIGATION: |
Summary of asset retirement obligation activity | The following table summarizes the asset retirement obligation activity for the years ended December 31, 2021 and 2020 (in millions):
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2022 |  | 2021
Balance as of January 1 | ​ | $ | 562.9 | ​ | $ | 545.0
Changes in estimates | ​ |  | (10.8) | ​ |  | —
Additions | ​ |  | 11.9 | ​ |  | ​
Closure payments | ​ |  | (7.1) | ​ |  | (5.6)
Accretion expense | ​ |  | 28.4 | ​ |  | 23.5
Balance as of December 31, | ​ | $ | 585.3 | ​ | $ | 562.9

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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/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI https://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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