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Summary of Share-Based Performance-Earned Restricted Stock Activity (Detail) (Share-based performance-earned restricted stock ("PERS"), USD $)
12 Months Ended
May 31, 2012
May 31, 2011
May 31, 2010
Share-based performance-earned restricted stock ("PERS")
     
Weighted-average grant-date fair value      
Beginning balance $ 17.29    
Shares granted $ 21.49 $ 19.31 $ 18.96
Shares forfeited $ 20.14    
Shares vested $ 14.49    
Ending balance $ 20.18 $ 17.29  
Shares      
Beginning balance 1,035,000    
Shares granted 468,000    
Shares forfeited (14,000)    
Shares vested (419,000)    
Ending balance 1,070,000 1,035,000