Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Components of property, plant and equipment
Land	$ 29,928	$ 23,380
Land improvements and leasehold improvements	77,191	71,817
Buildings and storage facilities	303,482	275,059
Machinery and equipment	375,028	333,559
Construction in progress	32,871	29,620
Property, plant and equipment, gross	818,500	733,435
Less: accumulated depreciation	363,240	321,959
Property, plant and equipment, net	$ 455,260	$ 411,476