Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2011
Components of property, plant and equipment
Land	$ 19,505	$ 17,655	$ 15,424
Land improvements and leasehold improvements	52,536	47,958	45,634
Buildings and storage facilities	172,354	150,461	142,864
Machinery and equipment	243,216	191,833	186,245
Software	11,204	10,861	10,603
Construction in progress	33,613	13,006	6,696
Property, plant and equipment, gross	532,428	431,774	407,466
Less accumulated depreciation and amortization	266,153	256,687	253,824
Property, plant and equipment, net	$ 266,275	$ 175,087	$ 153,642