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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Schedule of Change in Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Mar. 31, 2019
Dec. 31, 2018
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]          
Asset retirement obligation, beginning of period $ 19,104 $ 18,280 $ 18,280    
Revision of estimate 0   (662)    
Accretion of liabilities 513 459 1,835    
Settlements 0   (349)    
Asset retirement obligation, end of period 19,617   19,104    
Asset Retirement Obligation [Abstract]          
Current       $ 270 $ 270
Non-current       19,347 18,834
Asset retirement obligation, end of period $ 19,104 $ 18,280 $ 18,280 $ 19,617 $ 19,104