Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015
Property, plant and equipment
Property, plant and equipment, gross	$ 35,412	$ 35,849
Less: Accumulated depreciation & amortization	(25,158)	(24,724)
Property, plant and equipment, net	10,254	11,125
Mineral properties
Property, plant and equipment
Property, plant and equipment, gross	9,630	9,630
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	4,377	4,377
Mining equipment and machinery
Property, plant and equipment
Property, plant and equipment, gross	16,854	16,998
Other furniture and equipment
Property, plant and equipment
Property, plant and equipment, gross	841	841
Asset retirement cost
Property, plant and equipment
Property, plant and equipment, gross	$ 992	$ 1,285