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Stock-Based Compensation (Narrative) (Details) - USD ($)
$ / shares in Units, shares in Millions, $ in Millions
12 Months Ended
Apr. 30, 2020
Apr. 30, 2019
Apr. 30, 2018
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Stock-based compensation expenses $ 28.0 $ 23.8 $ 22.0
Tax benefit of stock-based compensation 7.2 6.1 6.9
Tax benefit from exercise of stock options $ 5.9 3.4 $ 15.3
Shares reserved for future awards under stock-based compensation plans (in shares) 12.8    
Maximum contractual term 10 years    
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
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 $ 22.1 17.9 $ 22.6
Unrecognized compensation cost $ 31.0    
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 $ 0.2    
Weighted-average period of recognition (years) 4 months    
Intrinsic value of options exercised $ 0.1 $ 0.4 $ 18.9
Minimum | 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    
Percentage of performance-based share units that ultimately vest 0.00%    
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 20000.00%