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ASSET RETIREMENT OBLIGATION: (Tables) | 3 Months Ended
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Mar. 31, 2025
ASSET RETIREMENT OBLIGATION: |
Summary of asset retirement obligation activity | The following table summarizes the asset retirement obligation activity for the three-month periods ended March 31, 2025 and 2024 (in millions):
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2025 |  | 2024
Balance as of January 1 | ​ | $ | 546.1 | ​ | $ | 612.5
Changes in estimates | ​ |  | (19.3) | ​ |  | (2.3)
Closure payments | ​ |  | — | ​ |  | —
Accretion expense | ​ |  | 6.0 | ​ |  | 7.6
Balance as of March 31, | ​ | $ | 532.9 | ​ | $ | 617.8
​

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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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