Business Segments (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Revenues	$ 32,603	$ 31,440	$ 114,052	$ 96,321
Operating profit (loss)	8,663	10,547	34,197	28,070
Expenditures for property, plant and equipment	7,297	5,450	13,264	14,632
Depreciation, depletion and amortization	4,044	3,815	12,095	10,935
Operating segments \| Coal Mining
Segment Reporting Information [Line Items]
Revenues	18,799	18,583	58,119	57,040
Operating profit (loss)	7,341	9,814	19,639	26,409
Expenditures for property, plant and equipment	2,291	2,162	6,542	6,588
Depreciation, depletion and amortization	3,102	3,097	9,252	9,030
Operating segments \| NAMining
Segment Reporting Information [Line Items]
Revenues	8,993	9,092	30,496	28,372
Operating profit (loss)	(391)	281	(842)	1,072
Expenditures for property, plant and equipment	4,971	3,122	6,213	6,353
Depreciation, depletion and amortization	588	441	1,699	1,263
Operating segments \| Minerals Management
Segment Reporting Information [Line Items]
Revenues	5,022	3,902	25,950	11,244
Operating profit (loss)	3,900	2,770	22,358	8,926
Expenditures for property, plant and equipment	0	0	291	1,182
Depreciation, depletion and amortization	324	250	1,057	562
Unallocated Items
Segment Reporting Information [Line Items]
Revenues	52	0	726	0
Operating profit (loss)	(2,286)	(2,318)	(7,115)	(8,337)
Expenditures for property, plant and equipment	35	166	218	509
Depreciation, depletion and amortization	30	27	87	80
Eliminations
Segment Reporting Information [Line Items]
Revenues	(263)	(137)	(1,239)	(335)
Operating profit (loss)	$ 99	$ 0	$ 157	$ 0

The aggregate expense recognized in the current period that allocates the cost of tangible assets, intangible assets, or depleting assets to periods that benefit from use of the assets.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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