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Plant, Equipment, and Mine Development, Net - Narrative (Details) - USD ($)
$ in Thousands
2 Months Ended 3 Months Ended 9 Months Ended
Feb. 28, 2021
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Property, Plant and Equipment [Line Items]            
Capitalized interest   $ 0 $ 561 $ 654 $ 895  
Plant, equipment and mine development, gross   99,099   99,099   $ 85,256
Mine development            
Property, Plant and Equipment [Line Items]            
Additions to property, plant and equipment       5,900    
Plant, equipment and mine development, gross   6,673   6,673   756
Construction in progress and other            
Property, Plant and Equipment [Line Items]            
Additions to property, plant and equipment $ 3,200          
Capitalized interest 700          
Plant, equipment and mine development, gross $ 34,100 $ 36,529   $ 36,529   $ 33,185