Stock-Based Compensation Plans - Option Transactions (Details) (USD $)	12 Months Ended
	May 31, 2013	May 31, 2012	May 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]
Shares Under Option, Outstanding at Beginning of Period	2,145,570	1,972,441	1,764,516
Shares Under Option, Granted		389,850	358,450
Shares Under Option, Exercised	(560,097)	(105,269)	(103,280)
Shares Under Option, Canceled	(76,940)	(111,452)	(47,245)
Shares Under Option, Outstanding at End of Period	1,508,533	2,145,570	1,972,441
Options exercisable (in shares)	878,423	1,160,420	1,040,521
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Roll Forward]
Weighted-Average Option Price, Outstanding at Beginning of Period	$ 38.54	$ 39.58	$ 38.54
Weighted-Average Option Price, Granted		$ 29.75	$ 40.22
Weighted-Average Option Price, Exercised	$ 32.90	$ 21.44	$ 22.51
Weighted-Average Option Price, Canceled	$ 37.72	$ 42.31	$ 43.13
Weighted-Average Option Price, Outstanding at End of Period	$ 40.68	$ 38.54	$ 39.58
Weighted-average exercise price (in USD per share)	$ 45.95	$ 42.59	$ 41.34