Acquisitions (Details 3) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Schedule of unaudited proforma information
Revenue - As reported	$ 1,583,536	$ 1,573,295	$ 1,563,860	$ 1,536,276	$ 1,541,094	$ 1,570,720	$ 1,513,344	$ 1,425,376	$ 6,256,967	$ 6,050,534	$ 5,136,435
Revenue - Pro forma										6,118,104	5,538,824
Net earnings - As reported	53,844	75,091	46,723	34,321	48,095	56,524	40,033	25,125	209,979	169,777	118,170
Net earnings - Pro forma										$ 184,849	$ 149,501
Net earnings per common share:
Basic - As reported (USD per share)	$ 1.05	$ 1.47	$ 0.92	$ 0.67	$ 0.94	$ 1.10	$ 0.78	$ 0.49	$ 4.11	$ 3.31	$ 2.25
Basic - Pro forma (USD per share)										$ 3.60	$ 2.85
Diluted - As reported (USD per share)	$ 1.05	$ 1.47	$ 0.91	$ 0.67	$ 0.93	$ 1.10	$ 0.77	$ 0.48	$ 4.09	$ 3.28	$ 2.25
Diluted - Pro forma (USD per share)										$ 3.58	$ 2.84