Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Components of property, plant and equipment
Land	$ 29,579	$ 22,388	$ 23,566
Land improvements and leasehold improvements	68,384	69,127	71,236
Buildings and storage facilities	280,226	284,820	298,077
Machinery and equipment	377,202	373,127	382,321
Construction in progress	37,456	7,502	7,372
Property, plant and equipment, gross	792,847	756,964	782,572
Less: accumulated depreciation	384,272	372,287	359,530
Property, plant and equipment, net	$ 408,575	$ 384,677	$ 423,042