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Note 3 - Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Property, plant, and equipment    
Less: Accumulated depreciation and amortization $ (73,494) $ (56,242)
Total property, plant and equipment, net 181,675 180,455
Plant and Equipment    
Property, plant, and equipment    
Property, plant and equipment, gross 161,566 155,173
Construction in Progress    
Property, plant, and equipment    
Property, plant and equipment, gross 4,890 7,245
Capitalized mine development cost    
Property, plant, and equipment    
Property, plant and equipment, gross 88,713 74,279
Coal Properties    
Property, plant, and equipment    
Capitalized amounts related to coal reserves at properties where the Company is not currently engaged in mining operations $ 14,700 $ 15,400