Property, Plant and Equipment - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment
Less: Accumulated depreciation	$ (200,734)	$ (197,038)
Property, plant and equipment, net	88,247	82,111
Land
Property, Plant and Equipment
Property, plant and equipment gross	1,130	2,330
Buildings and improvements
Property, Plant and Equipment
Property, plant and equipment gross	64,201	63,621
Machinery and equipment
Property, Plant and Equipment
Property, plant and equipment gross	$ 223,650	$ 213,198