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Business Segments (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2022
Jun. 30, 2021
Jun. 30, 2022
Jun. 30, 2021
Segment Reporting Information [Line Items]        
Revenues $ 61,369 $ 45,896 $ 116,392 $ 91,001
Operating profit (loss) 29,683 8,670 44,627 17,000
Expenditures for property, plant and equipment and acquisition of mineral interests 20,269 11,251 24,918 16,127
Depreciation, depletion and amortization 6,488 5,617 12,615 11,202
Operating segments | Coal Mining        
Segment Reporting Information [Line Items]        
Revenues 26,602 20,689 47,564 42,631
Operating profit (loss) 21,175 7,627 28,527 15,784
Expenditures for property, plant and equipment and acquisition of mineral interests 6,280 3,115 8,000 4,732
Depreciation, depletion and amortization 4,388 4,073 8,426 8,228
Operating segments | NAMining        
Segment Reporting Information [Line Items]        
Revenues 22,814 19,860 44,218 37,799
Operating profit (loss) 1,257 1,698 2,528 2,355
Expenditures for property, plant and equipment and acquisition of mineral interests 6,561 2,952 8,381 5,818
Depreciation, depletion and amortization 1,493 984 2,960 1,935
Operating segments | Minerals Management        
Segment Reporting Information [Line Items]        
Revenues 11,962 5,608 24,716 11,108
Operating profit (loss) 13,073 4,173 24,701 8,408
Expenditures for property, plant and equipment and acquisition of mineral interests 116 5,105 949 5,498
Depreciation, depletion and amortization 543 522 1,121 969
Unallocated Items        
Segment Reporting Information [Line Items]        
Revenues 617 910 809 1,053
Operating profit (loss) (5,952) (4,795) (11,391) (9,568)
Expenditures for property, plant and equipment and acquisition of mineral interests 7,312 79 7,588 79
Depreciation, depletion and amortization 64 38 108 70
Eliminations        
Segment Reporting Information [Line Items]        
Revenues (626) (1,171) (915) (1,590)
Operating profit (loss) $ 130 $ (33) $ 262 $ 21