Earnings per Share (Narrative) (Details) (USD $)	12 Months Ended							0 Months Ended	12 Months Ended		0 Months Ended	12 Months Ended		1 Months Ended	12 Months Ended
	May 31, 2014	May 31, 2013	May 31, 2012	May 31, 2014 Employee payroll taxes due on restricted stock [Member]	May 31, 2013 Employee payroll taxes due on restricted stock [Member]	May 31, 2014 Common Stock [Member] Employee payroll taxes due on restricted stock [Member]	May 31, 2013 Common Stock [Member] Employee payroll taxes due on restricted stock [Member]	Oct. 11, 2011 October 18, 2011 [Member]	May 31, 2014 October 18, 2011 [Member]	May 31, 2013 October 18, 2011 [Member]	Jul. 30, 2013 July 30, 2013 [Member]	May 31, 2014 July 30, 2013 [Member]	May 31, 2013 July 30, 2013 [Member]	Jun. 30, 2014 July 30, 2013 [Member] Subsequent Event [Member] Common Stock [Member]	Jul. 30, 2014 July 30, 2013 [Member] Subsequent Event [Member] Common Stock [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Options granted excluded from the computation of diluted earnings per share (shares)	700,000	700,000	2,000,000
Additional amount authorized under share buyback program								$ 500,000,000.0			$ 500,000,000.0
Common stock purchased under a share buyback program (shares)	6,665,000	5,075,000		200,000	200,000				3,324,000	5,075,000		3,341,000	0	700,000	4,100,000
Average price of common stock purchased under a share buyback program (dollars per share)	$ 54.31	$ 40.97				$ 50.45	$ 38.04		$ 48.87	$ 40.97		$ 59.72	$ 0.00	$ 62.14	$ 60.15
Aggregate purchase price of common stock under a share buyback program	$ 361,960,000	$ 207,932,000		$ 8,600,000	$ 7,700,000				$ 162,460,000	$ 207,932,000		$ 199,500,000	$ 0	$ 45,700,000	$ 245,200,000