Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Property, plant and equipment
Property, plant and equipment, Gross	$ 256,537	$ 256,487
Accumulated depreciation	(198,758)	(195,795)
Total property, plant and equipment, net	57,779	60,692
Manufacturing equipment [Member]
Property, plant and equipment
Property, plant and equipment, Gross	176,730	174,353
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	53,915	53,792
Office furniture and research equipment [Member]
Property, plant and equipment
Property, plant and equipment, Gross	$ 17,791	$ 20,241