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Property, Plant and Equipment, Net (Property, Plant and Equipment) (Details) - USD ($)
$ in Thousands
Dec. 31, 2024
Dec. 31, 2023
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, at cost $ 457,847 $ 452,158
Less allowances for depreciation, depletion, amortization and impairment 198,390 228,256
Property, plant and equipment, net 259,457 223,902
Coal lands and real estate    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, at cost 70,766 58,353
Mineral interests    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, at cost 69,148 68,150
Plant and equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, at cost $ 317,933 $ 325,655