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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Narrative (Details) - USD ($) $ in Thousands | 9 Months Ended |  |
--- | --- | --- | ---
Sep. 30, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Schedule of Asset Retirement Obligations [Line Items] |  |  |
Inflation rate | 2.00% | 2.00% |
Asset retirement obligation | $ 17,337 | $ 17,033 | $ 8,573
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% |  |
Canyon Project |  |  |
Schedule of Asset Retirement Obligations [Line Items] |  |  |
Asset retirement obligation | $ 43,000 | $ 43,000 |

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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 + Details
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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 + Details
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- Definition 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 + Details
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