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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, 2013
May 31, 2012
May 31, 2011
Share-based performance-earned restricted stock ("PERS")
     
Weighted-Average Grant-Date Fair Value      
Beginning balance $ 20.18    
Shares granted $ 26.22 $ 21.49 $ 19.31
Shares forfeited $ 20.62    
Shares vested $ 18.98    
Ending balance $ 22.31 $ 20.18  
Shares      
Beginning balance 1,070,000    
Shares granted 495,000    
Shares forfeited (19,000)    
Shares vested (92,000)    
Ending Balance 1,454,000 1,070,000