Business Segments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Revenues	$ 128,432	$ 140,990
Operating profit (loss)	13,448	38,820
Expenditures for property, plant and equipment and acquisition of mineral interests	44,368	24,664
Depreciation, depletion and amortization	18,114	16,240
Operating Segments | Coal Mining customer
Segment Reporting Information [Line Items]
Revenues	72,088	68,701
Operating profit (loss)	25,436	34,120
Expenditures for property, plant and equipment and acquisition of mineral interests	14,825	15,092
Depreciation, depletion and amortization	14,213	12,409
Operating Segments | NAMining customer
Segment Reporting Information [Line Items]
Revenues	42,392	42,823
Operating profit (loss)	1,872	(564)
Expenditures for property, plant and equipment and acquisition of mineral interests	13,862	8,824
Depreciation, depletion and amortization	2,470	2,223
Operating Segments | Minerals Management
Segment Reporting Information [Line Items]
Revenues	14,721	30,119
Operating profit (loss)	3,493	25,721
Expenditures for property, plant and equipment and acquisition of mineral interests	15,474	517
Depreciation, depletion and amortization	1,308	1,362
Unallocated Items
Segment Reporting Information [Line Items]
Revenues	2,133	790
Operating profit (loss)	(17,256)	(20,713)
Expenditures for property, plant and equipment and acquisition of mineral interests	207	231
Depreciation, depletion and amortization	123	246
Eliminations
Segment Reporting Information [Line Items]
Revenues	(2,902)	(1,443)
Operating profit (loss)	$ (97)	$ 256