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Stock Based Compensation Plans (Details) (USD $)
In Millions, except Per Share data
12 Months Ended
May 31, 2011
April 2004 Grants [Member]
 
Broad-based stock option grants  
Options granted 1.7
Exercise price per share $ 37.72
Options outstanding, Ending balance, Shares 0.7
Vesting schedule 25% each April in 2005 through 2008
October 2006 Grants [Member]
 
Broad-based stock option grants  
Options granted 2.0
Exercise price per share $ 37.32
Options outstanding, Ending balance, Shares 1.2
Vesting schedule 20% each October in 2007 through 2011