Summary Of Quarterly Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary Of Quarterly Data [Abstract]
Revenue	$ 307,201	$ 300,954	$ 317,862	$ 292,672	$ 283,757	$ 269,628	$ 281,482	$ 268,525	$ 1,218,689	$ 1,103,392	$ 1,031,633
Gross profit	158,881	158,667	174,033	154,925	144,771	142,207	149,284	142,361	646,506	578,623	526,281
Operating income	55,299	56,096	71,298	53,532	50,804	49,814	54,835	48,428	236,225	203,881	175,969
Net income attributable to stockholders	$ 38,010	$ 38,507	$ 48,657	$ 36,612	$ 36,371	$ 34,694	$ 37,193	$ 33,026	$ 161,786	$ 141,284	$ 122,225
Earnings per share:
Basic	$ 0.68	$ 0.68	$ 0.85	$ 0.64	$ 0.63	$ 0.60	$ 0.64	$ 0.57	$ 2.85	$ 2.45	$ 2.08
Diluted	$ 0.67	$ 0.66	$ 0.83	$ 0.62	$ 0.62	$ 0.59	$ 0.62	$ 0.55	$ 2.78	$ 2.37	$ 2.01