Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Components of property, plant and equipment
Land	$ 30,138	$ 29,928	$ 23,380
Land improvements and leasehold improvements	77,531	77,191	73,651
Buildings and storage facilities	304,276	303,482	274,877
Machinery and equipment	377,879	375,028	340,109
Construction in progress	47,755	32,871	18,354
Property, plant and equipment, gross	837,579	818,500	730,371
Less: accumulated depreciation	374,918	363,240	332,137
Property, plant and equipment, net	$ 462,661	$ 455,260	$ 398,234