Quarterly Information (Details) (USD $) In Thousands, except Per Share data	3 Months Ended																12 Months Ended
	Jul. 31, 2011		Apr. 30, 2011		Jan. 31, 2011		Oct. 31, 2010		Jul. 31, 2010		Apr. 30, 2010		Jan. 31, 2010		Oct. 31, 2009		Jul. 31, 2011	Jul. 31, 2010	Jul. 31, 2009
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 215,443	[1]	$ 236,755	[1]	$ 207,380	[1]	$ 212,667	[1]	$ 190,468		$ 220,349		$ 176,601		$ 185,461		$ 872,246	$ 772,879	$ 743,082
Operating income	63,456	[1]	82,044	[1]	60,195	[1]	59,594	[1]	57,220		72,126		53,232		56,492		265,290	239,070	225,325
Income from continuing operations	62,645	[1]	80,350	[1]	60,717	[1]	60,163	[1]	57,687		71,584		53,172		57,052
Net income	$ 40,521	[1]	$ 50,136	[1]	$ 37,893	[1]	$ 37,823	[1]	$ 36,234		$ 44,390		$ 35,733		$ 35,270
Basic net income per share (in dollars per share)	$ 0.60	[1],[2]	$ 0.72	[1],[2]	$ 0.47	[1],[2]	$ 0.45	[1],[2]	$ 0.43	[2]	$ 0.53	[2]	$ 0.42	[2]	$ 0.42	[2]	$ 2.20	$ 1.80	$ 1.69
Diluted net income per share (in dollars per share)	$ 0.59	[1],[2]	$ 0.71	[1],[2]	$ 0.46	[1],[2]	$ 0.45	[1],[2]	$ 0.43	[2]	$ 0.52	[2]	$ 0.42	[2]	$ 0.42	[2]	$ 2.17	$ 1.78	$ 1.66

[1] Earnings per share were computed independently for each of the periods presented; therefore, the sum of the earnings per share amounts for the quarters may not equal the total for the year.

[2] Fiscal 2011 results are impacted from the adoption of ASU 2009-13.