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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

X

- Definition

Civil Penalties

+ References

No definition available.

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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

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X

- 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

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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