Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Property, plant and equipment
Property, plant and equipment, Gross	$ 260,508	$ 256,537
Accumulated depreciation	(204,254)	(198,758)
Total property, plant and equipment, net	56,254	57,779
Machinery and Equipment [Member]
Property, plant and equipment
Property, plant and equipment, Gross	184,155	180,851
Land [Member]
Property, plant and equipment
Property, plant and equipment, Gross	8,101	8,101
Buildings and improvements [Member]
Property, plant and equipment
Property, plant and equipment, Gross	54,313	53,915
Computer Equipment [Member]
Property, plant and equipment
Property, plant and equipment, Gross	11,953	11,742
Office Equipment [Member]
Property, plant and equipment
Property, plant and equipment, Gross	$ 1,986	$ 1,928