Stock-Based Compensation Restricted stock awards granted and outstanding under either the 2005 Equity Compensation Plan or under previously adopted plans (Details) (Restricted Stock [Member], USD $)	12 Months Ended
	May 31, 2012	May 31, 2011	May 31, 2010
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Outstanding, begining of period (in shares)	1,917,382	1,407,351	981,369
Outstanding, Weighted Average Grant Price, begining of period (in dollars per share)	$ 28.22	$ 27.45	$ 30.29
Granted (in shares)	452,267	712,721	597,514
Granted, Weighted Average Grant Price (in dollares per share)	$ 35.95	$ 31.59	$ 24.63
Canceled (in shares)	(188,685)	(66,754)	(53,278)
Canceled, Weighted Average Grant Price (in dollars per share)	$ 30.62	$ 25.54	$ 27.85
Vested (in shares)	(291,968)	(135,936)	(118,254)
Vested, Weighted Average Grant Price (in dollars per share)	$ 27.60	$ 39.26	$ 36.57
Outstanding, end of period (in shares)	1,888,996	1,917,382	1,407,351
Outstanding, Weighted Average Grant Price, end of period (in dollars per share)	$ 29.93	$ 28.22	$ 27.45