Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Components of property, plant and equipment
Land	$ 30,672	$ 29,928
Land improvements and leasehold improvements	79,631	77,191
Buildings and storage facilities	322,856	303,482
Machinery and equipment	392,418	375,028
Construction in progress	12,784	32,871
Property, plant and equipment, gross	838,361	818,500
Less: accumulated depreciation	388,309	363,240
Property, plant and equipment, net	$ 450,052	$ 455,260