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Share-Based Compensation - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended
May 31, 2013
May 31, 2012
May 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Awards outstanding to officers, directors and key employees 1,400,000    
Total number of shares cumulatively authorized to be issued under our stock incentive plans 7,120,000    
Share-based compensation $ 3.9 $ 4.4 $ 5.0
Tax benefit related to share-based compensation 3.0 1.5 0.6
Unrecognized compensation expense related to share-based compensation 7.7    
Remaining weighted-average period 2 years 8 months 12 days    
Weighted-average remaining contractual life of Options exercisable 3 years 6 months    
Stock Options
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation 0 0.8 2.4
Award vesting period 4 years    
Stock option year term 10 years    
Performance Shares
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation 0.6 0.5 0.4
Award vesting period 4 years    
Stock And Stock Units
     
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation $ 3.3 $ 3.0 $ 2.2
Award vesting period 4 years