Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	May 31, 2012	Feb. 29, 2012	Nov. 30, 2011	Aug. 31, 2011	May 31, 2011	Feb. 28, 2011	Nov. 30, 2010	Aug. 31, 2010	May 31, 2012	May 31, 2011	May 31, 2010
Quarterly Financial Data [Abstract]
Revenue	$ 1,053,582	$ 1,012,112	$ 1,019,126	$ 1,017,180	$ 1,012,087	$ 937,827	$ 936,566	$ 923,904	$ 4,102,000	$ 3,810,384	$ 3,547,339
Gross Margin	443,868	425,903	429,797	439,040	433,047	391,921	390,648	393,671	1,738,608	1,609,287	1,497,817
Net income	$ 78,614	$ 76,035	$ 74,350	$ 68,638	$ 70,776	$ 59,070	$ 55,866	$ 61,277	$ 297,637	$ 246,989	$ 215,620
Basic earnings per share (in dollars per share)	$ 0.60	$ 0.58	$ 0.57	$ 0.52	$ 0.49	$ 0.41	$ 0.38	$ 0.40	$ 2.27	$ 1.68	$ 1.40
Diluted earnings per share (in dollars per share)	$ 0.60	$ 0.58	$ 0.57	$ 0.52	$ 0.49	$ 0.41	$ 0.38	$ 0.40	$ 2.27	$ 1.68	$ 1.40
Weighted average number of shares outstanding (in shares)	128,788	129,735	129,727	131,309	143,317	145,303	145,511	152,164	129,891	146,586	152,858