Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Components of property, plant and equipment
Land	$ 29,739	$ 22,388
Land improvements and leasehold improvements	68,826	69,127
Buildings and storage facilities	284,998	284,820
Machinery and equipment	393,640	373,127
Construction in progress	102,394	7,502
Property, plant and equipment, gross	879,597	756,964
Less: accumulated depreciation	402,886	372,287
Property, plant and equipment, net	$ 476,711	$ 384,677