Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Revenues	$ 45,896	$ 35,355	$ 91,001	$ 72,999
Operating profit (loss)	8,670	4,482	17,000	12,062
Expenditures for property, plant and equipment and acquisition of mineral interests	11,251	7,441	16,127	12,799
Depreciation, depletion and amortization	5,617	4,624	11,202	9,168
Operating segments | Coal Mining
Segment Reporting Information [Line Items]
Revenues	23,063	21,573	46,802	42,501
Operating profit (loss)	8,542	7,498	17,226	14,683
Expenditures for property, plant and equipment and acquisition of mineral interests	3,115	3,844	4,732	4,667
Depreciation, depletion and amortization	4,127	3,615	8,334	7,158
Operating segments | NAMining
Segment Reporting Information [Line Items]
Revenues	17,486	12,048	33,628	23,672
Operating profit (loss)	783	544	913	1,275
Expenditures for property, plant and equipment and acquisition of mineral interests	2,952	3,163	5,818	7,186
Depreciation, depletion and amortization	930	652	1,829	1,298
Operating segments | Minerals Management
Segment Reporting Information [Line Items]
Revenues	5,608	1,987	11,108	7,228
Operating profit (loss)	4,173	510	8,408	4,777
Expenditures for property, plant and equipment and acquisition of mineral interests	5,105	276	5,498	739
Depreciation, depletion and amortization	522	327	969	654
Unallocated Items
Segment Reporting Information [Line Items]
Revenues	910	327	1,053	353
Operating profit (loss)	(4,795)	(4,158)	(9,568)	(8,718)
Expenditures for property, plant and equipment and acquisition of mineral interests	79	158	79	207
Depreciation, depletion and amortization	38	30	70	58
Eliminations
Segment Reporting Information [Line Items]
Revenues	(1,171)	(580)	(1,590)	(755)
Operating profit (loss)	$ (33)	$ 88	$ 21	$ 45