Property, Plant and Equipment, Net (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2012
Property, plant and equipment
Property, plant and equipment, gross	$ 35,415	$ 35,849
Less: Accumulated depreciation & amortization	(25,573)	(24,724)
Property, plant and equipment, net	9,842	11,125
Mineral properties
Property, plant and equipment
Property, plant and equipment, gross	9,614	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	4,381	4,377
Mining equipment and machinery
Property, plant and equipment
Property, plant and equipment, gross	16,868	16,998
Other furniture and equipment
Property, plant and equipment
Property, plant and equipment, gross	842	841
Asset retirement cost
Property, plant and equipment
Property, plant and equipment, gross	$ 992	$ 1,285
Velardena properties | Asset retirement cost
Property, plant and equipment
Property, plant and equipment, net			$ 1,900