Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018	Mar. 31, 2018
Components of property, plant and equipment
Land	$ 39,552	$ 29,739	$ 29,915
Land improvements and leasehold improvements	82,681	68,826	69,320
Buildings and storage facilities	337,631	284,998	285,084
Machinery and equipment	481,454	393,640	377,563
Construction in progress	151,895	102,394	15,116
Property, plant and equipment, gross	1,093,213	879,597	776,998
Less: accumulated depreciation	421,408	402,886	383,235
Property, plant and equipment, net	$ 671,805	$ 476,711	$ 393,763