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ASSET RETIREMENT OBLIGATION: (Tables) | 6 Months Ended
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Jun. 30, 2024
ASSET RETIREMENT OBLIGATION: |
Summary of asset retirement obligation activity | The following table summarizes the asset retirement obligation activity for the six-month period ended June 30, 2024 and 2023 (in millions):
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2024 |  | 2023
Balance as of January 1 | ​ | $ | 612.6 | ​ | $ | 585.3
Changes in estimates | ​ |  | (7.7) | ​ |  | —
Closure payments | ​ |  | — | ​ |  | (0.3)
Accretion expense | ​ |  | 14.3 | ​ |  | 12.9
Balance as of June 30, | ​ | $ | 619.2 | ​ | $ | 597.9
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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 -Topic 410 -SubTopic 20 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147481850/410-20-50-1 + Details
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