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COMMITMENTS AND CONTINGENCIES - (Narrative) (Details) - USD ($)
$ in Thousands
1 Months Ended
Apr. 30, 2014
Jun. 30, 2017
Dec. 31, 2016
Dec. 31, 2015
Other Commitments [Line Items]        
Asset retirement obligation   $ 17,182 $ 17,033 $ 8,573
White Mesa Mill        
Other Commitments [Line Items]        
Civil penalties $ 38      
Canyon Project        
Other Commitments [Line Items]        
Loss contingency, estimate of possible loss   22,810 23,180  
Asset retirement obligation   $ 43,000 $ 43,000