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Stock-Based Compensation (Narrative) (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
12 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2016
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Stock-based compensation expenses $ 22.0 $ 19.3 $ 23.5
Document Fiscal Year Focus 2018    
Tax benefit of stock-based compensation $ 6.9 6.0 9.5
Tax benefit from exercise of stock options $ 15.3 5.9 20.9
Shares reserved for future awards under stock-based compensation plans 14.6    
2003 Long-Term Executive Compensation Plan      
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Vesting period (in years), minimum 3 years    
Nonvested Shares      
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Fair value of shares vesting during period $ 22.6 20.3 28.8
Unrecognized compensation cost $ 30.5    
Weighted-average period of recognition (years) 2 years    
Stock Options      
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Unrecognized compensation cost $ 1.1    
Weighted-average period of recognition (years) 2 years    
Intrinsic value of options exercised $ 18.9 $ 1.0 $ 11.7
Minimum | 2003 Long-Term Executive Compensation Plan      
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Percentage of performance-based share units that ultimately vest 0.00%    
Vesting period (in years), minimum 3 years    
Maximum | 2003 Long-Term Executive Compensation Plan      
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Percentage of performance-based share units that ultimately vest 250.00%    
Stock Options For Management And Director      
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Weighted average fair value (in dollars per share) $ 5.02 $ 3.31 $ 5.28