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ASSET RETIREMENT OBLIGATIONS - Activity (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Asset retirement and mine closing liability      
Balance at the beginning of the period $ 127,898 $ 137,514  
Accretion expense 3,688 4,033 $ 4,087
Payments (1,383) (1,769)  
Allocation of liability associated with acquisitions, mine development and change in assumptions 896 (11,880)  
Balance at the end of the period $ 131,099 $ 127,898 $ 137,514