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COMMITMENTS AND CONTINGENCIES - (Narrative) (Details) - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Other Commitments [Line Items]      
Asset retirement obligation $ 19,431 $ 18,280 $ 17,033
Canyon Project      
Other Commitments [Line Items]      
Loss contingency, estimate of possible loss 21,560 22,130  
Asset retirement obligation $ 43,170 $ 43,460