Inflation rate

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- Definition

[Line Items] for Schedule of Asset Retirement Obligations [Table]

+ References

No definition available.

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- Definition

The carrying amount of a liability for an asset retirement obligation. An asset retirement obligation is a legal obligation associated with the disposal or retirement of a tangible long-lived asset that results from the acquisition, construction or development, or the normal operations of a long-lived asset, except for certain obligations of lessees.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 410
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849

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- Definition

Risk-free interest rate assumption used in valuing an instrument.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)
-URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258

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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Asset Retirement Obligations [Line Items]
Inflation rate	2.00%
Asset retirement obligation	$ 19,431	$ 18,280	$ 17,033
Minimum
Schedule of Asset Retirement Obligations [Line Items]
Fair value assumptions, risk free interest rate	9.50%
Maximum
Schedule of Asset Retirement Obligations [Line Items]
Fair value assumptions, risk free interest rate	11.50%
Canyon Project
Schedule of Asset Retirement Obligations [Line Items]
Asset retirement obligation	$ 43,170	$ 43,460