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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Narrative (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Schedule of Asset Retirement Obligations [Line Items]
Inflation rate	2.00%	2.00%
Undiscounted decommissioning liability	$ 41,540	$ 32,300
2016 to 2020
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	5,190
2021 to 2025
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	9,410
2026 to 2030
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	2,690
2031 to 2035
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	8,170
2036 to 2038
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	$ 16,080
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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