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Business Segments (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Segment Reporting Information [Line Items]    
Revenues $ 45,105 $ 37,644
Operating profit (loss) 8,330 7,580
Expenditures for property, plant and equipment and acquisition of mineral interests 4,876 5,358
Depreciation, depletion and amortization 5,585 4,544
Operating segments | Coal Mining    
Segment Reporting Information [Line Items]    
Revenues 23,739 20,928
Operating profit (loss) 8,684 7,185
Expenditures for property, plant and equipment and acquisition of mineral interests 1,617 823
Depreciation, depletion and amortization 4,207 3,543
Operating segments | NAMining    
Segment Reporting Information [Line Items]    
Revenues 16,142 11,624
Operating profit (loss) 130 731
Expenditures for property, plant and equipment and acquisition of mineral interests 2,866 4,023
Depreciation, depletion and amortization 899 646
Operating segments | Minerals Management    
Segment Reporting Information [Line Items]    
Revenues 5,500 5,241
Operating profit (loss) 4,235 4,267
Expenditures for property, plant and equipment and acquisition of mineral interests 393 463
Depreciation, depletion and amortization 447 327
Unallocated Items    
Segment Reporting Information [Line Items]    
Revenues 143 26
Operating profit (loss) (4,773) (4,560)
Expenditures for property, plant and equipment and acquisition of mineral interests 0 49
Depreciation, depletion and amortization 32 28
Eliminations    
Segment Reporting Information [Line Items]    
Revenues (419) (175)
Operating profit (loss) $ 54 $ (43)