Business Combinations (Details 1) (USD $) In Thousands, except Per Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010
Unaudited condensed pro forma information
Revenue	$ 898,528	$ 811,316
Operating income	63,061	48,778
Net income	$ 36,309	$ 27,021
Earnings per share:
Basic earnings per share	$ 1.84	$ 1.39
Diluted earnings per share	$ 1.82	$ 1.38