Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 35,355	$ 41,352	$ 72,999	$ 81,449
Goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	20,976	21,879	41,260	37,964
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	14,379	19,473	31,739	43,485
Operating segments | Coal Mining
Disaggregation of Revenue [Line Items]
Revenues	21,573	22,570	42,501	39,320
Operating segments | NAMining
Disaggregation of Revenue [Line Items]
Revenues	12,048	10,728	23,672	21,503
Operating segments | Minerals Management
Disaggregation of Revenue [Line Items]
Revenues	1,987	8,242	7,228	20,928
Unallocated Items
Disaggregation of Revenue [Line Items]
Revenues	327	131	353	674
Eliminations
Disaggregation of Revenue [Line Items]
Revenues	$ (580)	$ (319)	$ (755)	$ (976)