XML 70 R54.htm IDEA: XBRL DOCUMENT v3.10.0.1
COMMITMENTS AND CONTINGENCIES - (Narrative) (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2016
Other Commitments [Line Items]      
Asset retirement obligation $ 19,046 $ 18,280 $ 17,033
Canyon Project      
Other Commitments [Line Items]      
Loss contingency, estimate of possible loss 21,590 22,130  
Asset retirement obligation $ 43,310 $ 43,460