Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 32,295	$ 32,603	$ 105,294	$ 114,052
Goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	19,871	18,049	61,131	56,012
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	12,424	14,554	44,163	58,040
Operating segments | Coal Mining
Disaggregation of Revenue [Line Items]
Revenues	20,395	18,799	62,896	58,119
Operating segments | NAMining
Disaggregation of Revenue [Line Items]
Revenues	9,443	8,993	33,115	30,496
Operating segments | Minerals Management
Disaggregation of Revenue [Line Items]
Revenues	2,722	5,022	9,950	25,950
Unallocated Items
Disaggregation of Revenue [Line Items]
Revenues	904	52	1,257	726
Eliminations
Disaggregation of Revenue [Line Items]
Revenues	$ (1,169)	$ (263)	$ (1,924)	$ (1,239)