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Share-Based Compensation - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
9 Months Ended
Feb. 28, 2013
Feb. 29, 2012
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 7,020,000  
Share-based compensation $ 3.0 $ 3.5
Tax benefit related to share-based compensation 3.0 1.2
Unrecognized compensation expense related to share-based compensation 8.7  
Remaining weighted-average period 2 years 10 months 24 days  
Weighted-average remaining contractual life of Options exercisable 3 years 8 months 12 days  
Stock Options
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation 0 0.8
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 0.4 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 $ 2.6 $ 2.3
Award vesting period 4 years