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Share-Based Compensation - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
6 Months Ended
Nov. 30, 2012
Nov. 30, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Awards outstanding to officers, directors and key employees 1,700,000  
Total number of shares cumulatively authorized to be issued under our stock incentive plans 7,020,000  
Share-based compensation $ 2.1 $ 2.6
Tax benefit related to share-based compensation 2.0 0.6
Unrecognized compensation expense related to share-based compensation 9.6  
Remaining weighted-average period 3 years  
Weighted-average remaining contractual life of Options exercisable 3 years 10 months 24 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 0.3 0.2
Award vesting period 4 years  
Stock And Stock Units
   
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation $ 1.8 $ 1.6
Award vesting period 4 years