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COMMITMENTS AND CONTINGENCIES - (Narrative) (Details) - USD ($) $ in Thousands | 1 Months Ended |  |  |
--- | --- | --- | --- | ---
Apr. 30, 2014 | Mar. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Other Commitments [Line Items] |  |  |  |
Asset retirement obligation |  | $ 18,716 | $ 18,280 | $ 17,033
White Mesa Mill |  |  |  |
Other Commitments [Line Items] |  |  |  |
Civil penalties | $ 38 |  |  |
Canyon Project |  |  |  |
Other Commitments [Line Items] |  |  |  |
Loss contingency, estimate of possible loss |  | 21,570 | 22,130 |
Asset retirement obligation |  | $ 43,420 | $ 43,460 |

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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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- Definition Reflects the estimated amount of loss from the specified contingency as of the balance sheet date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 4 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14435-108349 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 9 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82911808&loc=d3e14557-108349 + Details
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