Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, plant and equipment
Property, plant and equipment, gross	$ 34,128	$ 34,751
Less: Accumulated depreciation & amortization	(25,988)	(25,516)
Property, plant and equipment, net	8,140	9,235
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	4,246	4,386
Mining equipment and machinery
Property, plant and equipment
Property, plant and equipment, gross	15,989	16,351
Other furniture and equipment
Property, plant and equipment
Property, plant and equipment, gross	958	952
Asset retirement cost
Property, plant and equipment
Property, plant and equipment, gross	$ 865	$ 992