Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015	Sep. 30, 2015
Components of property, plant and equipment
Land	$ 28,473	$ 29,928	$ 30,285
Land improvements and leasehold improvements	82,908	77,191	76,414
Buildings and storage facilities	319,950	303,482	301,125
Machinery and equipment	393,178	375,028	368,338
Construction in progress	21,284	32,871	21,044
Property, plant and equipment, gross	845,793	818,500	797,206
Less: accumulated depreciation	385,546	363,240	354,884
Property, plant and equipment, net	$ 460,247	$ 455,260	$ 442,322