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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Narrative (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Schedule of Asset Retirement Obligations [Line Items]
Inflation rate	2.00%	2.00%
Undiscounted decommissioning liability	$ 43,000	$ 43,000
2017 to 2020
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	6,450
2021 to 2025
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	11,640
2026 to 2030
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	4,060
2031 to 2035
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	13,400
2036 to 2038
Schedule of Asset Retirement Obligations [Line Items]
Mine reclamation and closing liability, noncurrent	$ 7,900
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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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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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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