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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Narrative (Details) $ in Thousands | Sep. 30, 2021 USD ($) | Dec. 31, 2020 USD ($) | Dec. 31, 2019 USD ($)
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Schedule of Asset Retirement Obligations [Line Items] |  |  |
Inflation rate | 2.00% |  |
Asset retirement obligation | $ 14,021 | $ 13,038 | $ 18,972
Canyon Project |  |  |
Schedule of Asset Retirement Obligations [Line Items] |  |  |
Asset retirement obligation | $ 42,290 | $ 41,950 |
Measurement Input, Risk Free Interest Rate | Minimum |  |  |
Schedule of Asset Retirement Obligations [Line Items] |  |  |
Fair value assumptions, risk free interest rate | 0.095 |  |
Measurement Input, Risk Free Interest Rate | Maximum |  |  |
Schedule of Asset Retirement Obligations [Line Items] |  |  |
Fair value assumptions, risk free interest rate | 0.115 |  |

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