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COMMITMENTS AND CONTINGENCIES - (Narrative) (Details) - USD ($)
$ in Thousands
Mar. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Other Commitments [Line Items]      
Asset retirement obligation $ 19,617 $ 19,104 $ 18,280
Canyon Project      
Other Commitments [Line Items]      
Loss contingency, estimate of possible loss 19,680 19,650  
Asset retirement obligation $ 41,320 $ 41,320