Property, Plant and Equipment, Net (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2015	Dec. 31, 2014
Property, plant and equipment
Property, plant and equipment, gross	$ 49,521	$ 50,255
Less: Accumulated depreciation and amortization	(22,199)	(21,224)
Property, plant and equipment, net	27,322	29,031
Mineral Properties
Property, plant and equipment
Property, plant and equipment, gross	22,198	22,397
Exploration Properties
Property, plant and equipment
Property, plant and equipment, gross	2,743	2,743
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	4,378
Mining equipment and machinery
Property, plant and equipment
Property, plant and equipment, gross	17,460	17,694
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	$ 1,702	$ 2,002