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Note 3 - Property, Plant and Equipment - Schedule of Property, Plant and Equipment (Details) - USD ($)
$ in Thousands
Mar. 31, 2020
Dec. 31, 2019
Property, plant, and equipment    
Less: Accumulated depreciation and amortization $ (40,332) $ (35,330)
Total property, plant and equipment, net 181,896 178,202
Plant and Equipment    
Property, plant, and equipment    
Property, plant and equipment, gross 145,347 142,773
Construction in Progress    
Property, plant, and equipment    
Property, plant and equipment, gross 14,144 11,986
Capitalized mine development cost    
Property, plant, and equipment    
Property, plant and equipment, gross 62,737 58,773
Coal Properties    
Property, plant, and equipment    
Capitalized amounts related to coal reserves at properties where the Company is not currently engaged in mining operations $ 14,600 $ 12,700