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ASSET RETIREMENT OBLIGATIONS AND RESTRICTED CASH - Schedule of Change in Asset Retirement Obligation (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Dec. 31, 2018
Jun. 30, 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     151   (662)    
Accretion of liabilities $ 481 $ 458 994 917 1,835    
Settlements     0   (349)    
Asset retirement obligation, end of period 20,249   20,249   19,104    
Asset Retirement Obligation [Abstract]              
Current           $ 32 $ 270
Non-current           20,217 18,834
Asset retirement obligation, end of period $ 20,249   $ 19,104 $ 18,280 $ 18,280 $ 20,249 $ 19,104