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INCOME AND MINING TAXES - Tax Effects of Temporary Differences (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Deferred tax assets:        
Net operating loss carryforward $ 66,085 $ 57,667    
Mineral properties 66,038 60,299    
Other temporary differences 30,999 14,356    
Total gross deferred tax assets 163,122 132,322    
Less: valuation allowance (154,298) (121,212) $ (124,153) $ (123,648)
Net deferred tax assets 8,824 11,110    
Deferred tax liabilities:        
Acquired mineral property interests (12,637) (16,024)    
Total deferred tax liabilities (12,637) (16,024)    
Total net deferred tax liability (3,813) $ (4,914)    
Valuation allowances $ 33,100