Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2024	Mar. 31, 2023
Segment Reporting Information [Line Items]
Revenues	$ 53,289	$ 50,141
Cost of sales	46,271	46,784
Earnings of unconsolidated operations	13,307	13,824
Operating expenses	15,568	15,367
Operating profit (loss)	4,757	1,814
Expenditures for property, plant and equipment and acquisition of mineral interests	14,483	7,879
Depreciation, depletion and amortization	5,692	7,019
Operating segments | Coal Mining
Segment Reporting Information [Line Items]
Revenues	15,545	20,653
Cost of sales	20,943	25,878
Earnings of unconsolidated operations	12,007	12,466
Operating expenses	7,026	6,928
Operating profit (loss)	(417)	313
Expenditures for property, plant and equipment and acquisition of mineral interests	1,794	2,686
Depreciation, depletion and amortization	2,214	4,240
Operating segments | NAMining
Segment Reporting Information [Line Items]
Revenues	24,483	20,633
Cost of sales	21,671	19,241
Earnings of unconsolidated operations	1,365	1,358
Operating expenses	1,822	1,920
Operating profit (loss)	2,355	830
Expenditures for property, plant and equipment and acquisition of mineral interests	5,818	4,423
Depreciation, depletion and amortization	2,256	1,886
Operating segments | Minerals Management
Segment Reporting Information [Line Items]
Revenues	10,401	8,285
Cost of sales	1,364	1,052
Earnings of unconsolidated operations	(65)	0
Operating expenses	1,042	1,189
Operating profit (loss)	7,930	6,044
Expenditures for property, plant and equipment and acquisition of mineral interests	136	344
Depreciation, depletion and amortization	993	811
Unallocated Items
Segment Reporting Information [Line Items]
Revenues	3,262	1,191
Cost of sales	2,712	1,214
Operating expenses	5,678	5,330
Operating profit (loss)	(5,128)	(5,353)
Expenditures for property, plant and equipment and acquisition of mineral interests	6,735	426
Depreciation, depletion and amortization	229	82
Eliminations
Segment Reporting Information [Line Items]
Revenues	(402)	(621)
Cost of sales	(419)	(601)
Operating profit (loss)	$ 17	$ (20)