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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, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Dec. 31, 2019
Jun. 30, 2020
Dec. 31, 2019
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]              
Asset retirement obligation, beginning of period     $ 18,972 $ 19,104 $ 19,104    
Revision of estimate     0   (2,063)    
Accretion of liabilities $ 478 $ 481 956 994 1,931    
Asset retirement obligation, end of period 19,928   19,928   18,972    
Asset Retirement Obligation [Abstract]              
Current           $ 46 $ 46
Non-current           19,882 18,926
Asset retirement obligation, end of period $ 19,928   $ 18,972 $ 19,104 $ 18,972 $ 19,928 $ 18,972