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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Narrative (Details) - USD ($) $ in Thousands | Sep. 30, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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Schedule of Asset Retirement Obligations [Line Items] |  |  |
Inflation rate | 2.00% |  |
Asset retirement obligation | $ 20,733 | $ 19,104 | $ 18,280
Canyon Project |  |  |
Schedule of Asset Retirement Obligations [Line Items] |  |  |
Asset retirement obligation | $ 41,730 | $ 41,320 |
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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