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ASSET RETIREMENT OBLIGATION: (Tables)

12 Months Ended

Dec. 31, 2017

ASSET RETIREMENT OBLIGATION:

Summary of asset retirement obligation activity

        The following table summarizes the asset retirement obligation activity for years ended December 31, 2017 and 2016 (in millions):

		2017			2016
Balance as of January 1		$	216.5		$	190.9
Changes in estimates			(11.6	)		—
Additions			—			10.5
Closure payments			(0.3	)		(2.0	)
Accretion expense			17.9			17.1
​	​	​	​	​	​	​	​
Balance as of December 31,		$	222.5		$	216.5
​	​	​	​	​	​	​	​
​	​	​	​	​	​	​	​

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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://www.xbrl.org/2003/role/presentationRef
-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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