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Stock-Based Compensation (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended
Jul. 31, 2018
Jul. 31, 2017
Stock-based Compensation [Abstract]    
Share-based compensation expense (benefit) $ 3.9 $ (1.5)
Restricted Stock [Member]    
Restricted stock data for awards granted by the Company [Abstract]    
Awards granted (in shares) 230 387
Weighted average fair value of grant (in dollars per share) $ 66.55 $ 51.35
Performance-based Stock Awards [Member]    
Stock-based Compensation [Abstract]    
Period for achievement of performance-based targets 3 years