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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Narrative (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Schedule of Asset Retirement Obligations [Line Items] |  |
Inflation rate | 2.00% |
Undiscounted decommissioning liability | $ 41,950 | $ 41,750
Revision of estimate | 7,845 | $ 2,063
Change in estimated undiscounted cash flow | 200 |
Change in estimated timing of future reclamation activities | $ 7,650 |
Minimum |  |
Schedule of Asset Retirement Obligations [Line Items] |  |
Risk Free Rate | 9.50% |
Maximum |  |
Schedule of Asset Retirement Obligations [Line Items] |  |
Risk Free Rate | 11.50% |

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- Definition Amount of increase (decrease) in the asset retirement obligation from changes in the amount or timing of the estimated cash flows associated with the settlement of the obligation. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 35 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=6392603&loc=d3e7123-110846 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 410 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c)(4) -URI http://asc.fasb.org/extlink&oid=6392692&loc=d3e7535-110849 + Details
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