Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015	Jun. 30, 2015
Components of property, plant and equipment
Land	$ 28,472	$ 29,928	$ 29,902
Land improvements and leasehold improvements	77,849	77,191	75,822
Buildings and storage facilities	290,528	303,482	296,035
Machinery and equipment	374,107	375,028	356,759
Construction in progress	51,672	32,871	21,803
Property, plant and equipment, gross	822,628	818,500	780,321
Less: accumulated depreciation	375,361	363,240	343,247
Property, plant and equipment, net	$ 447,267	$ 455,260	$ 437,074