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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, 2019
Sep. 30, 2018
Sep. 30, 2019
Sep. 30, 2018
Dec. 31, 2018
Sep. 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 $ 484 $ 459 1,478 1,376 1,835    
Settlements     0   (349)    
Asset retirement obligation, end of period 20,733   20,733   19,104    
Asset Retirement Obligation [Abstract]              
Current           $ 32 $ 270
Non-current           20,701 18,834
Asset retirement obligation, end of period $ 20,733   $ 19,104 $ 18,280 $ 18,280 $ 20,733 $ 19,104