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Share-Based Payments (Details 2) - $ / shares
12 Months Ended
Apr. 30, 2015
Apr. 30, 2014
Apr. 30, 2013
Restricted Shares and Deferred Stock Units [Member]      
Summary of the Company's Restricted shares Deferred stock units and performance units      
Beginning Balance, Restricted Shares and Deferred Stock Units and Performance Units 839,188    
Beginning Balance, Weighted Average Grant Date Fair Value, in USD per share $ 76.54    
Granted Restricted Shares and Deferred Stock Units and Performance Units 109,091 167,134 109,770
Granted Weighted Average Grant Date Fair Value, in USD per share $ 104.82 $ 101.08 $ 76.37
Converted Restricted Shares and Deferred Stock Units and Performance Units 101,020    
Converted Weighted Average Grant Date Fair Value, in USD per share $ 104.91    
Vested Restricted Shares and Deferred Stock Units and Performance Units (416,328)    
Vested Weighted Average Grant Date Fair Value, in USD per share $ 68.59    
Forfeited Restricted Shares and Deferred Stock Units and Performance Units (36,082)    
Forfeited Weighted Average Grant Date Fair Value, in USD per share $ 90.65    
Ending Balance, Restricted Shares and Deferred Stock Units and Performance Units 596,889 839,188  
Ending Balance, Weighted Average Grant Date Fair Value, in USD per share $ 91.21 $ 76.54  
Performance Units [Member]      
Summary of the Company's Restricted shares Deferred stock units and performance units      
Beginning Balance, Restricted Shares and Deferred Stock Units and Performance Units 101,020    
Beginning Balance, Weighted Average Conversion Date Fair Value, in USD per share $ 104.91    
Granted Restricted Shares and Deferred Stock Units and Performance Units 75,848 101,020 106,666
Granted Weighted Average Conversion Date Fair Value, in USD per share $ 111.41 $ 104.91 $ 100.54
Converted Restricted Shares and Deferred Stock Units and Performance Units (101,020)    
Converted Weighted Average Conversion Date Fair Value, in USD per share $ 104.91    
Vested Restricted Shares and Deferred Stock Units and Performance Units 0    
Vested Weighted Average Conversion Date Fair Value, in USD per share $ 0.00    
Forfeited Restricted Shares and Deferred Stock Units and Performance Units 0    
Forfeited Weighted Average Conversion Date Fair Value, in USD per Share $ 0.00    
Ending Balance, Restricted Shares and Deferred Stock Units and Performance Units 75,848 101,020  
Ending Balance, Weighted Average Conversion Date Fair Value, in USD per share $ 111.41 $ 104.91