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Property, Plant and Equipment, Net (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, at cost $ 382,224 $ 335,633
Less allowances for depreciation, depletion and amortization 164,272 142,466
Property, plant and equipment, net 217,952 193,167
Depreciation, depletion and amortization 23,100 19,500
Coal lands and real estate    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, at cost 60,277 52,011
Mineral interests    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, at cost 31,436 19,512
Plant and equipment    
Property, Plant and Equipment [Line Items]    
Property, plant and equipment, at cost $ 290,511 $ 264,110