Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2024	Jun. 30, 2023	Jun. 30, 2024	Jun. 30, 2023
Segment Reporting Information [Line Items]
Revenues	$ 52,345	$ 61,350	$ 105,634	$ 111,491
Cost of sales	45,327	54,943	91,598	101,727
Earnings of unconsolidated operations	13,592	11,084	26,899	24,908
Operating expenses (income)	13,244	15,741	28,812	31,108
Operating profit (loss)	7,366	1,750	12,123	3,564
Expenditures for property, plant and equipment and acquisition of mineral interests	7,918	6,256	22,401	14,135
Depreciation, depletion and amortization	7,007	7,090	12,699	14,109
Operating segments | Coal Mining
Segment Reporting Information [Line Items]
Revenues	14,996	26,343	30,541	46,996
Cost of sales	16,138	33,269	37,081	59,147
Earnings of unconsolidated operations	12,006	9,962	24,013	22,428
Operating expenses (income)	8,097	7,711	15,123	14,639
Operating profit (loss)	2,767	(4,675)	2,350	(4,362)
Expenditures for property, plant and equipment and acquisition of mineral interests	1,913	1,032	3,707	3,718
Depreciation, depletion and amortization	2,896	4,348	5,110	8,588
Operating segments | NAMining
Segment Reporting Information [Line Items]
Revenues	27,920	21,716	52,403	42,349
Cost of sales	24,254	18,884	45,925	38,125
Earnings of unconsolidated operations	1,448	1,122	2,813	2,480
Operating expenses (income)	2,029	1,740	3,851	3,660
Operating profit (loss)	3,085	2,214	5,440	3,044
Expenditures for property, plant and equipment and acquisition of mineral interests	5,168	4,507	10,986	8,930
Depreciation, depletion and amortization	2,434	1,855	4,690	3,741
Operating segments | Minerals Management
Segment Reporting Information [Line Items]
Revenues	5,593	9,171	15,994	17,456
Cost of sales	1,501	910	2,865	1,962
Earnings of unconsolidated operations	138	0	73	0
Operating expenses (income)	(3,361)	972	(2,319)	2,161
Operating profit (loss)	7,591	7,289	15,521	13,333
Expenditures for property, plant and equipment and acquisition of mineral interests	10	638	146	982
Depreciation, depletion and amortization	1,323	749	2,316	1,560
Unallocated Items
Segment Reporting Information [Line Items]
Revenues	4,566	4,628	7,828	5,819
Cost of sales	4,167	2,375	6,879	3,589
Operating expenses (income)	6,479	5,318	12,157	10,648
Operating profit (loss)	(6,080)	(3,065)	(11,208)	(8,418)
Expenditures for property, plant and equipment and acquisition of mineral interests	827	79	7,562	505
Depreciation, depletion and amortization	354	138	583	220
Eliminations
Segment Reporting Information [Line Items]
Revenues	(730)	(508)	(1,132)	(1,129)
Cost of sales	(733)	(495)	(1,152)	(1,096)
Operating profit (loss)	$ 3	$ (13)	$ 20	$ (33)