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FAIR VALUE ACCOUNTING - Narrative (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Impairment of plant and equipment and mineral properties $ 0 $ 0 $ 8,220,000
Impairment of goodwill 0 0 $ 47,730,000
Impairment of plant and equipment and mineral properties $ 0    
Uranerz Energy Corporation      
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]      
Impairment of goodwill   $ 47,730,000