Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024	Dec. 31, 2024
Segment Reporting Information [Line Items]
Revenues	$ 76,614	$ 61,656	$ 210,420	$ 167,290
Cost of sales	66,643	54,412	183,975	146,010
Earnings of unconsolidated operations	16,494	15,155	45,618	42,054
Operating expenses (income)	19,688	16,312	57,655	45,124
Operating profit (loss)	6,777	19,699	14,408	31,822
Expenditures for property, plant and equipment and acquisition of mineral interests	22,392	8,296	34,342	30,697
Depreciation, depletion and amortization	6,194	6,251	19,078	18,950
Total assets	637,633		637,633		$ 631,687
Operating segments | Utility Coal Mining
Segment Reporting Information [Line Items]
Revenues	19,654	17,706	67,519	48,247
Cost of sales	21,511	18,054	74,408	55,135
Earnings of unconsolidated operations	14,311	13,821	40,430	37,834
Operating expenses (income)	7,467	7,147	23,541	22,270
Operating profit (loss)	4,987	19,938	10,000	22,288
Expenditures for property, plant and equipment and acquisition of mineral interests	4,550	3,338	6,600	7,045
Depreciation, depletion and amortization	2,135	2,154	6,285	7,264
Total assets	113,276		113,276		125,301
Operating segments | Contract Mining
Segment Reporting Information [Line Items]
Revenues	45,611	32,326	107,860	84,729
Cost of sales	42,395	31,379	99,432	77,304
Earnings of unconsolidated operations	1,074	1,122	3,275	3,935
Operating expenses (income)	2,361	2,543	6,794	6,394
Operating profit (loss)	1,929	(474)	4,909	4,966
Expenditures for property, plant and equipment and acquisition of mineral interests	10,697	3,170	18,483	14,156
Depreciation, depletion and amortization	2,777	2,672	8,396	7,362
Total assets	203,325		203,325		204,889
Operating segments | Minerals and Royalties
Segment Reporting Information [Line Items]
Revenues	9,313	8,849	27,483	24,843
Cost of sales	1,121	1,286	4,351	4,151
Earnings of unconsolidated operations	1,111	213	1,916	286
Operating expenses (income)	1,335	1,588	3,968	(731)
Operating profit (loss)	7,968	6,188	21,080	21,709
Expenditures for property, plant and equipment and acquisition of mineral interests	4,252	708	5,059	854
Depreciation, depletion and amortization	935	1,092	3,688	3,408
Total assets	99,937		99,937		99,905
Unallocated Items
Segment Reporting Information [Line Items]
Revenues	2,958	3,745	9,581	11,573
Cost of sales	2,502	4,622	7,790	11,501
Earnings of unconsolidated operations	(2)	(1)	(3)	(1)
Operating expenses (income)	8,525	5,034	23,352	17,191
Operating profit (loss)	(8,071)	(5,912)	(21,564)	(17,120)
Expenditures for property, plant and equipment and acquisition of mineral interests	2,893	1,080	4,200	8,642
Depreciation, depletion and amortization	347	333	709	916
Total assets	221,095		221,095		$ 201,592
Eliminations
Segment Reporting Information [Line Items]
Revenues	(922)	(970)	(2,023)	(2,102)
Cost of sales	(886)	(929)	(2,006)	(2,081)
Operating profit (loss)	$ (36)	$ (41)	$ (17)	$ (21)