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Share-Based Payments (Details 2) - $ / shares
12 Months Ended
Apr. 30, 2017
Apr. 30, 2016
Apr. 30, 2015
Restricted Shares and Deferred Stock Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Beginning Balance, Restricted Shares and Deferred Stock Units and Performance Units 545,742    
Granted Restricted Shares and Deferred Stock Units and Performance Units 180,997 97,922 109,091
Converted Restricted Shares and Deferred Stock Units and Performance Units 121,936    
Vested Restricted Shares and Deferred Stock Units and Performance Units (243,561)    
Forfeited Restricted Shares and Deferred Stock Units and Performance Units (31,709)    
Ending Balance, Restricted Shares and Deferred Stock Units and Performance Units 573,405 545,742  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]      
Beginning Balance, Weighted Average Grant Date Fair Value (in dollars per share) $ 99.65    
Granted Weighted Average Grant Date Fair Value (in dollars per share) 133.92 $ 113.57 $ 104.82
Converted Weighted Average Grant Date Fair Value (in dollars per share) 132.46    
Vested Weighted Average Grant Date Fair Value (in dollars per share) 100.81    
Forfeited Weighted Average Grant Date Fair Value (in dollars per share) 118.97    
Ending Balance, Weighted Average Grant Date Fair Value (in dollars per share) $ 115.88 $ 99.65  
Performance Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]      
Beginning Balance, Restricted Shares and Deferred Stock Units and Performance Units 121,936    
Granted Restricted Shares and Deferred Stock Units and Performance Units 73,701 121,936 75,848
Converted Restricted Shares and Deferred Stock Units and Performance Units (121,936)    
Vested Restricted Shares and Deferred Stock Units and Performance Units 0    
Forfeited Restricted Shares and Deferred Stock Units and Performance Units 0    
Ending Balance, Restricted Shares and Deferred Stock Units and Performance Units 73,701 121,936  
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]      
Beginning Balance, Weighted Average Grant Date Fair Value (in dollars per share) $ 132.46    
Granted Weighted Average Grant Date Fair Value (in dollars per share) 126.80    
Converted Weighted Average Grant Date Fair Value (in dollars per share) 132.46    
Vested Weighted Average Grant Date Fair Value (in dollars per share) 0.00    
Forfeited Weighted Average Grant Date Fair Value (in dollars per share) 0.00    
Ending Balance, Weighted Average Grant Date Fair Value (in dollars per share) $ 126.80 $ 132.46