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Stock-Based Compensation, Performance-Based and Other Restricted Stock Activity (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
12 Months Ended
Apr. 30, 2018
Apr. 30, 2017
Apr. 30, 2016
Apr. 30, 2015
Restricted Stock Awards [Member]        
Restricted Shares [Roll Forward]        
Nonvested Shares at Beginning of Year (in shares) 913 915 752  
Granted (in shares) 525 509 289  
Change in Shares Due to Performance (in shares) (107) (67)    
Change in Shares Due to Performance (in shares)     86  
Vested and Issued (in shares) (318) (267) (154)  
Forfeited (in shares) (152) (177) (58)  
Nonvested Shares at End of Year (in shares) 861 913 915 752
Weighted Average Grant Date Value [Abstract]        
Nonvested Shares at Beginning of Year (in dollars per share) $ 51.85      
Granted (in dollars per share) 53.59      
Change in Shares Due to Performance (in dollars per share) 55.70      
Vested and Issued (in dollars per share) 49.47      
Forfeited (in dollars per share) 52.40      
Nonvested Shares at End of Year (in dollars per share) $ 53.22 $ 51.85    
Restricted Stock, Additional Disclosures [Abstract]        
Unrecognized share-based compensation expense $ 31.1      
Award vesting period 4 years      
Weighted average recognition period for unrecognized share-based compensation 2 years 2 months 12 days      
Total grant date fair value of restricted shares vested $ 15.7 $ 12.1 $ 7.2  
Restricted Stock Awards [Member] | Key Employees [Member] | Vesting on Fourth Anniversary Date Following Date of Grant [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting percentage       50.00%
Restricted Stock Awards [Member] | Key Employees [Member] | Vesting on Fifth Anniversary Date Following Date of Grant [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting percentage       50.00%
Restricted Stock Awards [Member] | Key Employees [Member] | Annual Vesting on Anniversary of Grant [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting percentage     25.00%  
Performance-based Restricted Stock Awards [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Period for achievement of performance-based targets 3 years      
Performance-based Restricted Stock Awards [Member] | Vesting on First Anniversary Date after Award Is Earned [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting percentage       50.00%
Performance-based Restricted Stock Awards [Member] | Vesting on Second Anniversary Date after Award Is Earned [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting percentage       50.00%
Performance-based Restricted Stock Awards [Member] | Vesting at End of Performance Cycle [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting percentage 50.00%      
Performance-based Restricted Stock Awards [Member] | Vesting on April 30th of Year Following Performance Cycle [Member]        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Award vesting percentage 50.00%