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Share-Based Payments (Details 1) (USD $)
12 Months Ended
Apr. 30, 2012
Apr. 30, 2011
Apr. 30, 2010
Deferred Compensation, Share-based Payments [Member]
     
Summary of the Company's Restricted shares Deferred shares Deferred stock units and performance units      
Beginning Balance, Restricted/Deferred Shares and Deferred Stock Units and Performance Units 1,157,266    
Beginning Balance, Weighted Average Grant Date Fair Value, in USD per share $ 49.39    
Granted Restricted/Deferred Shares and Deferred Stock Units and Performance Units 152,180 303,863 504,580
Converted Restricted/Deferred Shares and Deferred Stock Units and Performance Units 125,360    
Vested Restricted/Deferred Shares and Deferred Stock Units and Performance Units (430,831)    
Forfeited Restricted/Deferred Shares and Deferred Stock Units and Performance Units (12,985)    
Granted Weighted Average Grant Date Fair Value, in USD per share $ 78.32 $ 58.32 $ 44.63
Converted Weighted Average Grant Date Fair Value, in USD per share $ 77.53    
Vested Weighted Average Grant Date Fair Value, in USD per share $ 52.60    
Forfeited Weighted Average Grant Date Fair Value, in USD per share $ 52.91    
Ending Balance, Restricted/Deferred Shares and Deferred Stock Units and Performance Units 990,990 1,157,266  
Ending Balance, Weighted Average Grant Date Fair Value, in USD per share $ 55.95 $ 49.39  
Performance Units [Member]
     
Summary of the Company's Restricted shares Deferred shares Deferred stock units and performance units      
Beginning Balance, Restricted/Deferred Shares and Deferred Stock Units and Performance Units 125,360    
Beginning Balance, Weighted Average Grant Date Fair Value, in USD per share $ 77.53    
Granted Restricted/Deferred Shares and Deferred Stock Units and Performance Units 99,455 125,360 190,010
Converted Restricted/Deferred Shares and Deferred Stock Units and Performance Units (125,360)    
Vested Restricted/Deferred Shares and Deferred Stock Units and Performance Units 0    
Forfeited Restricted/Deferred Shares and Deferred Stock Units and Performance Units 0    
Granted Weighted Average Grant Date Fair Value, in USD per share $ 76.37 $ 77.53 $ 57.37
Converted Weighted Average Grant Date Fair Value, in USD per share $ 77.53    
Vested Weighted Average Grant Date Fair Value, in USD per share $ 0.00    
Forfeited Weighted Average Grant Date Fair Value, in USD per share $ 0.00    
Ending Balance, Restricted/Deferred Shares and Deferred Stock Units and Performance Units 99,455 125,360  
Ending Balance, Weighted Average Grant Date Fair Value, in USD per share $ 76.37 $ 77.53