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ASSET RETIREMENT OBLIGATION (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
ASSET RETIREMENT OBLIGATION    
Beginning balance at January 1 $ 5,759 $ 6,092
Asset retirement obligation 771 850
Settlement of asset retirement obligation (1,375)  
Retirement cost revaluation (1,235) (1,486)
Accretion expense 217 303
Ending balance at December 31 $ 4,137 $ 5,759