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Share-Based Compensation (Details) (USD $)
In Millions, except Share data in Thousands, unless otherwise specified
3 Months Ended 6 Months Ended
Oct. 31, 2011
Oct. 31, 2010
Oct. 31, 2011
Oct. 31, 2010
Share-Based Compensation [Abstract]        
Share-based compensation expense $ 4.2 $ 4.4 $ 7.7 $ 8.3
Restricted Stock [Member]
       
Share-based compensation data for awards granted by the Company [Abstract]        
Awards granted (in thousands)     256 256
Weighted average fair value of grant (in dollars per share) $ 49.55 $ 39.94 $ 49.55 $ 39.94
Stock Options [Member]
       
Share-based compensation data for awards granted by the Company [Abstract]        
Awards granted (in thousands)     411 413
Weighted average fair value of grant (in dollars per share)     $ 14.11 $ 11.97
Weighted-average Black Scholes fair value assumptions by stock option grants [Abstract]        
Expected life of options (in years)     7.3 7.7
Risk-free interest rate (in hundredths)     2.30% 2.70%
Expected volatility (in hundredths)     29.00% 28.90%
Expected dividend yield (in hundredths)     1.60% 1.60%
Fair value of common stock on grant date (in dollars per share) $ 49.55 $ 40.02 $ 49.55 $ 40.02