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Asset retirement obligations (“ARO”) (Tables)

12 Months Ended

Dec. 31, 2014

Asset Retirement Obligation Disclosure [Abstract]

Schedule of changes in asset retirement obligations

The reconciliation of the changes in the asset retirement obligations is presented below:

	Year ended December 31,
	2014			2013
Beginning balance, ARO liability	$	27,113		$	16,124
Additional ARO liability incurred	2,548			1,730
ARO liabilities settled	(4,731		)	(2,580		)
Accretion expense	1,517			1,733
Additional ARO liability from Sebree acquisition	—			10,106
Additional ARO liability from Mt. Holly acquisition	10,503			—
Ending balance, ARO liability	$	36,950		$	27,113

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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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