Property, Plant and Equipment (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013	Mar. 31, 2013
Components of property, plant and equipment
Land	$ 21,906	$ 21,801	$ 22,637
Land improvements and leasehold improvements	67,876	67,153	64,972
Buildings and storage facilities	234,109	231,976	219,500
Machinery and equipment	309,283	308,215	290,930
Software	13,403	13,351	13,464
Construction in progress	52,200	48,135	38,893
Property, plant and equipment, gross	698,777	690,631	650,396
Less: accumulated depreciation and amortization	312,645	303,173	286,089
Property, plant and equipment, net	$ 386,132	$ 387,458	$ 364,307