Accrued Warranty Costs (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Product warranty obligation
Balance at beginning of period	$ 5,370	$ 3,996
Amounts charged to costs and expenses	1,568	1,627
Costs incurred	(1,322)	(1,339)
Balance at end of period	$ 5,616	$ 4,284