Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 32,603	$ 31,440	$ 114,052	$ 96,321
Goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	18,049	17,975	56,012	55,170
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	14,554	13,465	58,040	41,151
Operating segments | Coal Mining
Disaggregation of Revenue [Line Items]
Revenues	18,799	18,583	58,119	57,040
Operating segments | NAMining
Disaggregation of Revenue [Line Items]
Revenues	8,993	9,092	30,496	28,372
Operating segments | Minerals Management
Disaggregation of Revenue [Line Items]
Revenues	5,022	3,902	25,950	11,244
Unallocated Items
Disaggregation of Revenue [Line Items]
Revenues	52	0	726	0
Eliminations
Disaggregation of Revenue [Line Items]
Revenues	$ (263)	$ (137)	$ (1,239)	$ (335)