Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	May 31, 2013	Feb. 28, 2013	Nov. 30, 2012	Aug. 31, 2012	May 31, 2012	Feb. 29, 2012	Nov. 30, 2011	Aug. 31, 2011	May 31, 2013	May 31, 2012	May 31, 2011
Quarterly Financial Data [Abstract]
Revenue	$ 1,129,086	$ 1,075,674	$ 1,060,386	$ 1,051,325	$ 1,053,582	$ 1,012,112	$ 1,019,126	$ 1,017,180	$ 4,316,471	$ 4,102,000	$ 3,810,384
Gross margin	467,215	441,941	432,036	445,875	443,868	425,903	429,797	439,040	1,787,067	1,738,608	1,609,287
Net income	$ 85,977	$ 74,705	$ 78,027	$ 76,733	$ 78,614	$ 76,035	$ 74,350	$ 68,638	$ 315,442	$ 297,637	$ 246,989
Basic earnings per share (dollars per share)	$ 0.69	$ 0.60	$ 0.63	$ 0.61	$ 0.60	$ 0.58	$ 0.57	$ 0.52	$ 2.53	$ 2.27	$ 1.68
Diluted earnings per share (dollars per share)	$ 0.69	$ 0.60	$ 0.63	$ 0.60	$ 0.60	$ 0.58	$ 0.57	$ 0.52	$ 2.52	$ 2.27	$ 1.68
Weighted average number of shares outstanding (shares)	122,392	123,120	124,185	126,110	128,788	129,735	129,727	131,309	123,956	129,891	146,586