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ASSET RETIREMENT OBLIGATION: (Tables)

6 Months Ended

Jun. 30, 2019

Summary of asset retirement obligation activity

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The following table summarizes the asset retirement obligation activity for the six months ended June 30, 2019 and 2018 (in millions):

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​	​	​	​	​	​	​
​		2019			2018
Balance as of January 1,	​	$	217.7	​	$	222.5
Changes in estimates	​		25.2	​		(15.6)
Closure payments	​		(0.4)	​		—
Accretion expense	​		6.2	​		6.2
Balance as of June 30,	​	$	248.7	​	$	213.1

​

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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

+ Details

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Period Type:	duration