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COMMITMENTS AND CONTINGENCIES - (Narrative) (Details) - USD ($)
$ in Thousands
Mar. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Other Commitments [Line Items]      
Asset retirement obligation $ 13,320 $ 13,038 $ 18,972
Canyon Project      
Other Commitments [Line Items]      
Loss contingency, estimate of possible loss 20,830 20,820  
Asset retirement obligation $ 42,290 $ 41,950