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Share-Based Compensation (Details) - USD ($)
$ / shares in Units, $ in Millions
3 Months Ended
Jul. 31, 2016
Jul. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation expense $ 0.2 $ 3.9
Restricted Stock [Member]    
Restricted stock data for awards granted by the Company [Abstract]    
Awards granted (in shares) 368,000 285,000
Weighted average fair value of grant (in dollars per share) $ 51.04 $ 55.99
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  
Stock Options [Member]    
Restricted stock data for awards granted by the Company [Abstract]    
Awards granted (in shares) 0 160,153
Weighted average fair value of grant (in dollars per share)   $ 14.77
Weighted-average Black-Scholes fair value assumptions by stock option grants [Abstract]    
Expected life of options   7 years 2 months 12 days
Risk-free interest rate   2.10%
Expected volatility   29.70%
Expected dividend yield   2.10%
Fair value of common stock on grant date (in dollars per share)   $ 55.99