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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Schedule of Change in Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2018
Dec. 31, 2017
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]          
Asset retirement obligation, beginning of period $ 18,280 $ 17,033      
Revision of estimate (662) 249 $ 4,186    
Accretion of liabilities 1,835 1,733 906    
Settlements (349) (735)      
Asset retirement obligation, end of period 19,104 18,280 17,033    
Asset Retirement Obligation [Abstract]          
Current       $ 270 $ 32
Non-current       18,834 18,248
Asset retirement obligation, end of period $ 18,280 $ 17,033 $ 17,033 $ 19,104 $ 18,280