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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Dec. 31, 2016
Schedule of Asset Retirement Obligations [Line Items]
Inflation rate	2.00%	2.00%
2017 - 2021
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	$ 8,210
2022 - 2026
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	11,220
2027 - 2031
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	3,650
2032 - 2036
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	11,950
2037 - 2039
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	$ 7,970
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%

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

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

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258

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Amount of a reclamation and mine closing liability that is associated with a legal obligation for the closure and reclamation of a mine including the removal of buildings, equipment, machinery and other physical remnants of mining, closure of tailings impoundments, leach pads and other mine features, and contouring, covering and revegetation of waste rock piles and other disturbed areas.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 450
-SubTopic 20
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 5.Y.Q2)
-URI http://asc.fasb.org/extlink&oid=27011672&loc=d3e149879-122751

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