Property, Plant and Equipment, net (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Property, plant and equipment
Property, plant and equipment, gross	$ 50,261	$ 50,521
Less: Accumulated depreciation and amortization	(18,816)	(18,146)
Property, plant and equipment, net	31,445	32,375
Mineral properties
Property, plant and equipment
Property, plant and equipment, gross	22,397	22,397
Exploration properties
Property, plant and equipment
Property, plant and equipment, gross	2,993	2,993
Royalty properties
Property, plant and equipment
Property, plant and equipment, gross	200	200
Buildings
Property, plant and equipment
Property, plant and equipment, gross	2,357	2,349
Mining equipment and machinery
Property, plant and equipment
Property, plant and equipment, gross	19,441	19,441
Other furniture and equipment
Property, plant and equipment
Property, plant and equipment, gross	871	1,054
Asset retirement cost
Property, plant and equipment
Property, plant and equipment, gross	$ 2,002	$ 2,087