Property, Plant and Equipment (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014	Jun. 30, 2014
Components of property, plant and equipment
Land	$ 29,902	$ 23,380	$ 22,055
Land improvements and leasehold improvements	75,822	71,817	68,003
Buildings and storage facilities	296,035	275,059	234,745
Machinery and equipment	356,759	333,559	314,427
Software	68,456	55,436	55,710
Construction in progress	21,803	29,620	18,362
Property, plant and equipment, gross	848,777	788,871	713,302
Less: accumulated depreciation and amortization	359,632	335,264	322,715
Property, plant and equipment, net	$ 489,145	$ 453,607	$ 390,587