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ASSET RETIREMENT OBLIGATION: (Tables) | 6 Months Ended
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Jun. 30, 2023
ASSET RETIREMENT OBLIGATION: |
Summary of asset retirement obligation activity | ​ The following table summarizes the asset retirement obligation activity for the first six months of 2023 and 2022 (in millions):
​ | ​ | ​ | ​ | ​ | ​ | ​
​ |  | 2023 |  | 2022
Balance as of January 1 | ​ | $ | 585.3 | ​ | $ | 562.9
Changes in estimates | ​ |  | — | ​ |  | 43.3
Closure payments | ​ |  | (0.3) | ​ |  | (3.6)
Accretion expense | ​ |  | 12.9 | ​ |  | 13.0
Balance as of June 30, | ​ | $ | 597.9 | ​ | $ | 615.6

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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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