Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011	Mar. 31, 2011
Components of property, plant and equipment
Land	$ 17,171	$ 17,655	$ 15,424
Land improvements and leasehold improvements	48,587	47,958	45,359
Buildings and storage facilities	153,666	150,461	142,017
Machinery and equipment	196,434	191,833	183,568
Software	10,949	10,861	10,549
Construction in progress	20,888	13,006	2,734
Property, plant and equipment, gross	447,695	431,774	399,651
Less accumulated depreciation and amortization	260,111	256,687	249,389
Property, plant and equipment, net	$ 187,584	$ 175,087	$ 150,262