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Share-Based Payments (Details 1) (USD $)
12 Months Ended
Apr. 30, 2011
Apr. 30, 2010
Apr. 30, 2009
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 1,078,722    
Beginning Balance, Weighted Average Grant Date Fair Value, in USD per share $ 44.74    
Granted Restricted/Deferred Shares and Deferred Stock Units and Performance Units 303,863 504,580 570,359
Converted Restricted/Deferred Shares and Deferred Stock Units and Performance Units 190,010    
Vested Restricted/Deferred Shares and Deferred Stock Units and Performance Units (373,522)    
Forfeited Restricted/Deferred Shares and Deferred Stock Units and Performance Units (41,807)    
Granted weighted average grant date fair value, in USD per share $ 58.32 $ 44.63 $ 42.29
Converted Weighted Average Grant Date Fair Value, in USD per share $ 57.37    
Vested Weighted Average Grant Date Fair Value, in USD per share $ 47.33    
Forfeited Weighted Average Grant Date Fair Value, [in USD per share] $ 49.08    
Ending Balance, Restricted/Deferred Shares and Deferred Stock Units and Performance Units 1,157,266 1,078,722  
Ending Balance, Weighted Average Grant Date Fair Value, in USD per share $ 49.39 $ 44.74  
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 190,010    
Beginning Balance, Weighted Average Grant Date Fair Value, in USD per share $ 57.37    
Granted Restricted/Deferred Shares and Deferred Stock Units and Performance Units 125,360 190,010 114,440
Converted Restricted/Deferred Shares and Deferred Stock Units and Performance Units (190,010)    
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 $ 77.53 $ 57.37 $ 43.44
Converted Weighted Average Grant Date Fair Value, in USD per share $ 57.37    
Vested Weighted Average Grant Date Fair Value, in USD per share $ 0    
Forfeited Weighted Average Grant Date Fair Value, [in USD per share] $ 0    
Ending Balance, Restricted/Deferred Shares and Deferred Stock Units and Performance Units 125,360 190,010  
Ending Balance, Weighted Average Grant Date Fair Value, in USD per share $ 77.53 $ 57.37