Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Property, plant and equipment
Property, plant and equipment, gross	$ 33,610	$ 33,625
Less: Accumulated depreciation and amortization	(28,139)	(27,594)
Property, plant and equipment, net	5,471	6,031
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	2,418	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	3,755
Mining equipment and machinery
Property, plant and equipment
Property, plant and equipment, gross	16,051	16,049
Other furniture and equipment
Property, plant and equipment
Property, plant and equipment, gross	885	884
Asset retirement cost
Property, plant and equipment
Property, plant and equipment, gross	$ 948	$ 866