Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment
Gross property, plant and equipment	$ 1,826,361	$ 1,793,728
Less: Accumulated depreciation	(1,342,913)	(1,323,828)
Property, plant and equipment, net	483,448	469,900
Land
Property, Plant and Equipment
Gross property, plant and equipment	10,515	10,621
Land improvements
Property, Plant and Equipment
Gross property, plant and equipment	15,932	16,156
Buildings and improvements
Property, Plant and Equipment
Gross property, plant and equipment	190,715	191,072
Machinery and equipment
Property, Plant and Equipment
Gross property, plant and equipment	1,566,706	1,538,166
Construction in progress
Property, Plant and Equipment
Gross property, plant and equipment	$ 42,493	$ 37,713