Inventories - Summary of inventory (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Inventory Disclosure [Abstract]
Raw materials	$ 16,120	$ 14,002
Work in progress	2,746	2,848
Finished goods	6,253	5,238
Supplies	916	868
Total Inventories	26,035	22,956
Inventory reserve	(74)	(61)
Net Inventories	$ 25,961	$ 22,895