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Share-Based Compensation (Details 2) (Performance Shares [Member], USD $)
In Thousands, except Per Share data, unless otherwise specified
3 Months Ended
Aug. 31, 2012
Aug. 31, 2011
Performance Shares [Member]
   
Transactions involving performance awards    
Stock and stock units, beginning of period, No. of Stock Units 64 61
Stock and stock units, beginning of period, Weighted Average Fair Value $ 21.86 $ 20.33
Changes during the period    
Granted, No. of Performance Awards      
Granted, Weighted Average Fair Value      
Vested and settled, No. of Performance Awards      
Vested and settled, Weighted Average Fair Value      
Cancelled, No. of Performance Awards      
Cancelled, Weighted Average Fair Value      
Stock and stock units, end of period, No. of Stock Units 64 61
Stock and stock units, end of period, Weighted Average Fair Value $ 21.86 $ 20.33