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Business Segments (Details) - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Sep. 30, 2021
Sep. 30, 2020
Segment Reporting Information [Line Items]        
Revenues $ 51,742 $ 32,295 $ 142,743 $ 105,294
Operating profit (loss) 27,592 9,398 44,592 21,460
Expenditures for property, plant and equipment and acquisition of mineral interests 19,407 7,003 35,534 19,802
Depreciation, depletion and amortization 5,796 4,876 16,998 14,044
Operating segments | Coal Mining        
Segment Reporting Information [Line Items]        
Revenues 23,682 20,395 70,484 62,896
Operating profit (loss) 23,121 11,174 40,347 25,857
Expenditures for property, plant and equipment and acquisition of mineral interests 5,646 3,150 10,378 7,817
Depreciation, depletion and amortization 4,323 3,793 12,657 10,951
Operating segments | NAMining        
Segment Reporting Information [Line Items]        
Revenues 17,693 9,443 51,321 33,115
Operating profit (loss) 312 244 1,225 1,519
Expenditures for property, plant and equipment and acquisition of mineral interests 13,309 3,220 19,127 10,406
Depreciation, depletion and amortization 1,014 723 2,843 2,021
Operating segments | Minerals Management        
Segment Reporting Information [Line Items]        
Revenues 10,607 2,722 21,715 9,950
Operating profit (loss) 9,454 1,673 17,862 6,450
Expenditures for property, plant and equipment and acquisition of mineral interests 450 633 5,948 1,372
Depreciation, depletion and amortization 423 327 1,392 981
Unallocated Items        
Segment Reporting Information [Line Items]        
Revenues 1,594 904 2,647 1,257
Operating profit (loss) (5,170) (3,623) (14,738) (12,341)
Expenditures for property, plant and equipment and acquisition of mineral interests 2 0 81 207
Depreciation, depletion and amortization 36 33 106 91
Eliminations        
Segment Reporting Information [Line Items]        
Revenues (1,834) (1,169) (3,424) (1,924)
Operating profit (loss) $ (125) $ (70) $ (104) $ (25)