Shareholders' Equity (Details) (USD $)	1 Months Ended	3 Months Ended		12 Months Ended
	Aug. 31, 2011	Feb. 28, 2011	Aug. 31, 2010	May 31, 2012	May 31, 2011
Stockholders' Equity Note [Abstract]
Stock repurchase program, authorized amount	$ 100,000,000
Repurchase of common stock (in shares)			344,847	2,290,059
Repurchase of common stock			13,000,000	99,604,000	12,980,000
Treasury stock acquired, average cost per share			$ 37.64	$ 43.49
Stock repurchase program, remaining authorized repurchase amount			13,000,000
Treasury stock retired, shares		344,847		2,290,059
Treasury stock retired, amount		$ 13,000,000		$ 99,600,000