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ASSET RETIREMENT OBLIGATION: (Tables) | 6 Months Ended
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Jun. 30, 2026
ASSET RETIREMENT OBLIGATION: |
Summary of asset retirement obligation activity | The following table summarizes the asset retirement obligation activity for the six-month periods ended June 30, 2026 and 2025 (in millions): ​
​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ ​ ​ | 2026 | ​ ​ ​ | 2025
Balance as of January 1 | ​ | $ | 471.1 | ​ | $ | 546.1
Changes in estimates | ​ |  | 9.2 | ​ |  | (48.5)
Closure payments | ​ |  | (2.4) | ​ |  | —
Accretion expense | ​ |  | 12.8 | ​ |  | 12.2
Balance as of June 30, | ​ | $ | 490.7 | ​ | $ | 509.9
​

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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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