Revenue Recognition (Disaggregation of Revenue) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 128,432	$ 140,990
Goods transferred at a point in time
Disaggregation of Revenue [Line Items]
Revenues	68,073	66,102
Services transferred over time
Disaggregation of Revenue [Line Items]
Revenues	60,359	74,888
Operating Segments | Coal Mining customer
Disaggregation of Revenue [Line Items]
Revenues	72,088	68,701
Operating Segments | NAMining
Disaggregation of Revenue [Line Items]
Revenues	42,392	42,823
Operating Segments | Minerals Management lessee
Disaggregation of Revenue [Line Items]
Revenues	14,721	30,119
Unallocated Items
Disaggregation of Revenue [Line Items]
Revenues	2,133	790
Eliminations
Disaggregation of Revenue [Line Items]
Revenues	$ (2,902)	$ (1,443)