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ASSET RETIREMENT OBLIGATION: (Tables)

12 Months Ended

Dec. 31, 2020

ASSET RETIREMENT OBLIGATION:

Summary of asset retirement obligation activity

The following table summarizes the asset retirement obligation activity for the years ended December 31, 2020 and 2019 (in millions):

​	​	​	​	​	​	​
​		2020			2019
Balance as of January 1	​	$	262.3	​	$	217.7
Changes in estimates	​		269.3	​		36.4
Closure payments	​		(1.2)	​		(5.3)
Accretion expense	​		14.6	​		13.5
Balance as of December 31,	​	$	545.0	​	$	262.3

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- References

No definition available.

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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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=6393242&loc=d3e13201-110859

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