Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property, plant and equipment
Property, plant and equipment, gross	$ 33,618	$ 34,127
Less: Accumulated depreciation & amortization	(26,774)	(27,018)
Property, plant and equipment, net	6,844	7,109
Mineral properties
Property, plant and equipment
Property, plant and equipment, gross	9,353	9,353
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	3,755	4,278
Mining equipment and machinery
Property, plant and equipment
Property, plant and equipment, gross	16,038	16,024
Other furniture and equipment
Property, plant and equipment
Property, plant and equipment, gross	888	888
Asset retirement cost
Property, plant and equipment
Property, plant and equipment, gross	$ 866	$ 866