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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Narrative (Details) - USD ($) $ in Thousands | 3 Months Ended |  |
--- | --- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Schedule of Asset Retirement Obligations [Line Items] |  |  |
Inflation rate | 2.00% |  |
Asset retirement obligation | $ 19,617 | $ 19,104 | $ 18,280
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 | $ 41,320 | $ 41,320 |

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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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