Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2013
Components of property, plant and equipment
Land	$ 22,055	$ 21,801	$ 22,637
Land improvements and leasehold improvements	68,003	67,153	65,625
Buildings and storage facilities	234,745	231,976	224,281
Machinery and equipment	314,427	308,215	295,723
Software	55,710	13,351	13,469
Construction in progress	18,362	48,135	44,146
Property, plant and equipment, gross	713,302	690,631	665,881
Less: accumulated depreciation and amortization	322,715	303,173	294,165
Property, plant and equipment, net	$ 390,587	$ 387,458	$ 371,716