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ASSET RETIREMENT OBLIGATIONS - Activity (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Asset retirement and mine closing liability      
Balance at the beginning of the period $ 137,514 $ 137,114  
Accretion expense 4,033 4,087 $ 3,926
Payments (1,769) (2,948)  
Allocation of liability associated with acquisitions, mine development and change in assumptions (11,880) (739)  
Balance at the end of the period $ 127,898 $ 137,514 $ 137,114