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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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- 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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