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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Schedule of Change in Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Sep. 30, 2018
Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]              
Asset retirement obligation, beginning of period     $ 18,280 $ 17,033 $ 17,033    
Revision of estimate     0   249    
Accretion of liabilities $ 459 $ 345 1,376 1,036 1,733    
Settlements     (225)   (735)    
Asset retirement obligation, end of period 19,431   19,431   18,280    
Asset Retirement Obligation [Abstract]              
Current           $ 32 $ 32
Non-current           19,399 18,248
Asset retirement obligation, end of period $ 19,431   $ 18,280 $ 17,033 $ 17,033 $ 19,431 $ 18,280