Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Components of property, plant and equipment
Land	$ 22,388	$ 30,672
Land improvements and leasehold improvements	69,127	79,631
Buildings and storage facilities	284,820	322,856
Machinery and equipment	373,127	392,418
Construction in progress	7,502	12,784
Property, plant and equipment, gross	756,964	838,361
Less: accumulated depreciation	372,287	388,309
Property, plant and equipment, net	$ 384,677	$ 450,052