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Stock-Based Compensation (Narrative) (Details) - USD ($)
shares in Millions, $ in Millions
12 Months Ended
Apr. 30, 2016
Apr. 30, 2015
Apr. 30, 2014
Deferred Compensation Arrangement with Individual, Excluding Share-based Payments and Postretirement Benefits [Line Items]      
Stock-based compensation expenses $ 23.5 $ 26.1 $ 20.1
Document Fiscal Year Focus 2016    
Tax benefit of stock-based compensation $ 9.5 9.9 7.6
Realized tax benefits $ 20.9 12.5 10.6
Shares reserved for future awards under stock-based compensation plans 8.7    
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 $ 28.8 14.3 8.6
Unrecognized compensation cost $ 32.1    
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 $ 11.7 $ 8.4 $ 13.6
Unrecognized compensation cost $ 0.4    
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