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Business Segments (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2023
Sep. 30, 2022
Sep. 30, 2023
Sep. 30, 2022
Segment Reporting Information [Line Items]        
Revenues $ 46,546 $ 61,793 $ 158,037 $ 178,185
Operating (loss) profit (6,267) 9,818 (2,703) 54,445
Expenditures for property, plant and equipment and acquisition of mineral interests 23,759 17,086 37,894 42,004
Depreciation, depletion and amortization 7,320 6,569 21,429 19,184
Operating segments | Coal Mining        
Segment Reporting Information [Line Items]        
Revenues 18,665 22,599 65,661 70,163
Operating (loss) profit (4,697) 6,089 (9,059) 34,616
Expenditures for property, plant and equipment and acquisition of mineral interests 1,469 3,141 5,187 11,141
Depreciation, depletion and amortization 4,336 4,257 12,924 12,683
Operating segments | NAMining        
Segment Reporting Information [Line Items]        
Revenues 21,722 22,962 64,071 67,180
Operating (loss) profit 866 (210) 3,910 2,318
Expenditures for property, plant and equipment and acquisition of mineral interests 21,450 604 30,380 8,985
Depreciation, depletion and amortization 2,058 1,585 5,799 4,545
Operating segments | Minerals Management        
Segment Reporting Information [Line Items]        
Revenues 5,747 16,172 23,203 40,888
Operating (loss) profit 3,610 10,616 16,943 35,317
Expenditures for property, plant and equipment and acquisition of mineral interests 776 11,397 1,758 12,346
Depreciation, depletion and amortization 768 660 2,328 1,781
Unallocated Items        
Segment Reporting Information [Line Items]        
Revenues 966 1,092 6,785 1,901
Operating (loss) profit (6,027) (6,780) (14,445) (18,171)
Expenditures for property, plant and equipment and acquisition of mineral interests 64 1,944 569 9,532
Depreciation, depletion and amortization 158 67 378 175
Eliminations        
Segment Reporting Information [Line Items]        
Revenues (554) (1,032) (1,683) (1,947)
Operating (loss) profit $ (19) $ 103 $ (52) $ 365