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Business Segments (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2020
Jun. 30, 2019
Jun. 30, 2020
Jun. 30, 2019
Segment Reporting Information [Line Items]        
Revenues $ 35,355 $ 41,352 $ 72,999 $ 81,449
Operating profit (loss) 4,482 9,161 12,062 25,534
Expenditures for property, plant and equipment 7,441 1,715 12,799 5,967
Depreciation, depletion and amortization 4,624 4,238 9,168 8,051
Operating segments | Coal Mining        
Segment Reporting Information [Line Items]        
Revenues 21,573 22,570 42,501 39,320
Operating profit (loss) 7,498 7,262 14,683 17,269
Expenditures for property, plant and equipment 3,844 1,511 4,667 4,251
Depreciation, depletion and amortization 3,615 3,276 7,158 6,150
Operating segments | NAMining        
Segment Reporting Information [Line Items]        
Revenues 12,048 10,728 23,672 21,503
Operating profit (loss) 544 (450) 1,275 (385)
Expenditures for property, plant and equipment 3,163 112 7,186 1,242
Depreciation, depletion and amortization 652 566 1,298 1,111
Operating segments | Minerals Management        
Segment Reporting Information [Line Items]        
Revenues 1,987 8,242 7,228 20,928
Operating profit (loss) 510 6,789 4,777 18,458
Expenditures for property, plant and equipment 276 50 739 291
Depreciation, depletion and amortization 327 367 654 733
Unallocated Items        
Segment Reporting Information [Line Items]        
Revenues 327 131 353 674
Operating profit (loss) (4,158) (4,732) (8,718) (9,866)
Expenditures for property, plant and equipment 158 42 207 183
Depreciation, depletion and amortization 30 29 58 57
Eliminations        
Segment Reporting Information [Line Items]        
Revenues (580) (319) (755) (976)
Operating profit (loss) $ 88 $ 292 $ 45 $ 58