Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012
Components of property, plant and equipment
Land	$ 22,637	$ 22,258	$ 19,505
Land improvements and leasehold improvements	65,625	63,013	52,536
Buildings and storage facilities	224,281	214,919	172,354
Machinery and equipment	295,723	287,896	243,216
Software	13,469	12,901	11,204
Construction in progress	44,146	34,965	33,613
Property, plant and equipment, gross	665,881	635,952	532,428
Less accumulated depreciation and amortization	294,165	277,074	266,153
Property, plant and equipment, net	$ 371,716	$ 358,878	$ 266,275