Property, Plant and Equipment - Summary of Property, Plant and Equipment (Details) - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Property, plant and equipment
Less: Accumulated depreciation	$ (205,503)		$ (200,734)
Property, plant and equipment, net	96,670		88,247
Depreciation	11,227	$ 10,201
Land
Property, plant and equipment
Property, plant and equipment gross	1,136		1,130
Buildings and improvements
Property, plant and equipment
Property, plant and equipment gross	68,883		64,201
Machinery and equipment
Property, plant and equipment
Property, plant and equipment gross	$ 232,154		$ 223,650