Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Segment Reporting Information [Line Items]
Revenues	$ 50,141	$ 55,023
Operating profit (loss)	1,814	14,944
Expenditures for property, plant and equipment and acquisition of mineral interests	7,879	4,649
Depreciation, depletion and amortization	7,019	6,127
Operating segments | Coal Mining
Segment Reporting Information [Line Items]
Revenues	20,653	20,962
Operating profit (loss)	313	7,352
Expenditures for property, plant and equipment and acquisition of mineral interests	2,686	1,720
Depreciation, depletion and amortization	4,240	4,038
Operating segments | NAMining
Segment Reporting Information [Line Items]
Revenues	20,633	21,404
Operating profit (loss)	830	1,271
Expenditures for property, plant and equipment and acquisition of mineral interests	4,423	1,820
Depreciation, depletion and amortization	1,886	1,467
Operating segments | Minerals Management
Segment Reporting Information [Line Items]
Revenues	8,285	12,754
Operating profit (loss)	6,044	11,628
Expenditures for property, plant and equipment and acquisition of mineral interests	344	833
Depreciation, depletion and amortization	811	578
Unallocated Items
Segment Reporting Information [Line Items]
Revenues	1,191	192
Operating profit (loss)	(5,353)	(5,439)
Expenditures for property, plant and equipment and acquisition of mineral interests	426	276
Depreciation, depletion and amortization	82	44
Eliminations
Segment Reporting Information [Line Items]
Revenues	(621)	(289)
Operating profit (loss)	$ (20)	$ 132