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Business Segments (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Jun. 30, 2019
Jun. 30, 2018
Segment Reporting Information [Line Items]        
Revenues $ 41,352 $ 33,681 $ 81,449 $ 64,881
Operating profit (loss) 9,161 7,802 25,534 17,523
Expenditures for property, plant and equipment 1,715 6,730 5,967 9,182
Depreciation, depletion and amortization 4,238 3,724 8,051 7,120
Operating segments | Coal Mining        
Segment Reporting Information [Line Items]        
Revenues 22,570 20,860 39,320 38,457
Operating profit (loss) 4,693 7,898 12,298 16,595
Expenditures for property, plant and equipment 1,623 3,850 5,493 4,426
Depreciation, depletion and amortization 3,276 3,092 6,150 5,933
Operating segments | NAMining        
Segment Reporting Information [Line Items]        
Revenues 10,728 9,067 21,503 19,280
Operating profit (loss) (483) 157 (451) 791
Expenditures for property, plant and equipment 0 2,557 0 3,231
Depreciation, depletion and amortization 566 427 1,111 822
Operating segments | Minerals Management        
Segment Reporting Information [Line Items]        
Revenues 8,242 3,866 20,928 7,342
Operating profit (loss) 6,789 3,212 18,458 6,156
Expenditures for property, plant and equipment 50 150 291 1,182
Depreciation, depletion and amortization 367 178 733 312
Unallocated Items        
Segment Reporting Information [Line Items]        
Revenues 131 0 674 0
Operating profit (loss) (2,130) (3,465) (4,829) (6,019)
Expenditures for property, plant and equipment 42 173 183 343
Depreciation, depletion and amortization 29 27 57 53
Eliminations        
Segment Reporting Information [Line Items]        
Revenues (319) (112) (976) (198)
Operating profit (loss) $ 292 $ 0 $ 58 $ 0