Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Property, plant and equipment
Property, plant and equipment, gross	$ 34,163	$ 34,128
Less: Accumulated depreciation & amortization	(26,531)	(25,988)
Property, plant and equipment, net	7,632	8,140
Mineral properties
Property, plant and equipment
Property, plant and equipment, gross	9,352	9,352
Exploration properties
Property, plant and equipment
Property, plant and equipment, gross	2,518	2,518
Royalty properties
Property, plant and equipment
Property, plant and equipment, gross	200	200
Buildings
Property, plant and equipment
Property, plant and equipment, gross	4,274	4,246
Mining equipment and machinery
Property, plant and equipment
Property, plant and equipment, gross	15,987	15,989
Other furniture and equipment
Property, plant and equipment
Property, plant and equipment, gross	966	958
Asset retirement cost
Property, plant and equipment
Property, plant and equipment, gross	$ 866	$ 865