Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Segment Reporting Information [Line Items]
Revenues	$ 61,793	$ 51,742	$ 178,185	$ 142,743
Operating profit (loss)	9,818	27,592	54,445	44,592
Expenditures for property, plant and equipment and acquisition of mineral interests	17,086	19,407	42,004	35,534
Depreciation, depletion and amortization	6,569	5,796	19,184	16,998
Operating segments | Coal Mining
Segment Reporting Information [Line Items]
Revenues	22,599	20,946	70,163	63,577
Operating profit (loss)	6,089	21,985	34,616	37,769
Expenditures for property, plant and equipment and acquisition of mineral interests	3,141	5,646	11,141	10,378
Depreciation, depletion and amortization	4,257	4,306	12,683	12,534
Operating segments | NAMining
Segment Reporting Information [Line Items]
Revenues	22,962	20,429	67,180	58,228
Operating profit (loss)	(210)	1,448	2,318	3,803
Expenditures for property, plant and equipment and acquisition of mineral interests	604	13,309	8,985	19,127
Depreciation, depletion and amortization	1,585	1,031	4,545	2,966
Operating segments | Minerals Management
Segment Reporting Information [Line Items]
Revenues	16,172	10,607	40,888	21,715
Operating profit (loss)	10,616	9,454	35,317	17,862
Expenditures for property, plant and equipment and acquisition of mineral interests	11,397	450	12,346	5,948
Depreciation, depletion and amortization	660	423	1,781	1,392
Unallocated Items
Segment Reporting Information [Line Items]
Revenues	1,092	1,594	1,901	2,647
Operating profit (loss)	(6,780)	(5,170)	(18,171)	(14,738)
Expenditures for property, plant and equipment and acquisition of mineral interests	1,944	2	9,532	81
Depreciation, depletion and amortization	67	36	175	106
Eliminations
Segment Reporting Information [Line Items]
Revenues	(1,032)	(1,834)	(1,947)	(3,424)
Operating profit (loss)	$ 103	$ (125)	$ 365	$ (104)