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Property, Plant and Equipment, Net
 12 Months Ended
Dec. 31, 2021
Property, Plant and Equipment, Net [Abstract]  
Property, Plant and Equipment, Net Property, Plant and Equipment, Net
Property, plant and equipment, net includes the following:
 December 31
 20212020
Coal lands and real estate$52,011 $50,887 
Mineral interests19,512 14,181 
Plant and equipment264,110 231,190 
Property, plant and equipment, at cost335,633 296,258 
Less allowances for depreciation, depletion and amortization142,466 123,841 
 $193,167 $172,417 
Total depreciation, depletion and amortization expense on property, plant and equipment was $19.5 million and $15.5 million during 2021 and 2020, respectively.