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Stock-Based Compensation (Narrative) (Details) - USD ($)
shares in Millions, $ in Millions
12 Months Ended
Apr. 30, 2017
Apr. 30, 2016
Apr. 30, 2015
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Stock-based compensation expenses $ 19.3 $ 23.5 $ 26.1
Document Fiscal Year Focus 2017    
Tax benefit of stock-based compensation $ 6.0 9.5 9.9
Employee Service Share-based Compensation, Tax Benefit from Exercise of Stock Options $ 5.9 20.9 12.5
Shares reserved for future awards under stock-based compensation plans 8.2    
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 $ 20.3 28.8 14.3
Unrecognized compensation cost $ 25.0    
Weighted-average period of recognition (years) 2 years    
Stock Options      
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Intrinsic value of options exercised $ 1.0 $ 11.7 $ 8.4
Unrecognized compensation cost $ 0.1    
Weighted-average period of recognition (years) 2 years    
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