Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2022	Mar. 31, 2021
Segment Reporting Information [Line Items]
Revenues	$ 55,023	$ 45,105
Operating profit (loss)	14,944	8,330
Expenditures for property, plant and equipment and acquisition of mineral interests	4,649	4,876
Depreciation, depletion and amortization	6,127	5,585
Operating segments | Coal Mining
Segment Reporting Information [Line Items]
Revenues	20,962	21,942
Operating profit (loss)	7,352	8,157
Expenditures for property, plant and equipment and acquisition of mineral interests	1,720	1,617
Depreciation, depletion and amortization	4,038	4,155
Operating segments | NAMining
Segment Reporting Information [Line Items]
Revenues	21,404	17,939
Operating profit (loss)	1,271	657
Expenditures for property, plant and equipment and acquisition of mineral interests	1,820	2,866
Depreciation, depletion and amortization	1,467	951
Operating segments | Minerals Management
Segment Reporting Information [Line Items]
Revenues	12,754	5,500
Operating profit (loss)	11,628	4,235
Expenditures for property, plant and equipment and acquisition of mineral interests	833	393
Depreciation, depletion and amortization	578	447
Unallocated Items
Segment Reporting Information [Line Items]
Revenues	192	143
Operating profit (loss)	(5,439)	(4,773)
Expenditures for property, plant and equipment and acquisition of mineral interests	276	0
Depreciation, depletion and amortization	44	32
Eliminations
Segment Reporting Information [Line Items]
Revenues	(289)	(419)
Operating profit (loss)	$ 132	$ 54