Joint Venture (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Summary of financial data for joint venture
Net sales	$ 304,039	$ 294,860	$ 996,347	$ 850,216
Cost of materials	218,015	221,690	722,663	631,588
Income before taxes	3,626	5,069	80	15,203
Net income	3,173	3,803	(4,105)	10,203
Joint venture [Member]
Summary of financial data for joint venture
Net sales	63,578	71,776	208,045	202,455
Cost of materials	52,705	57,743	172,370	163,453
Income before taxes	4,129	7,382	15,663	20,892
Net income	$ 2,716	$ 6,234	$ 12,198	$ 17,916