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Stock-Based Compensation (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended
Jul. 31, 2017
Jul. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation (benefit) expense $ (1.5) $ 0.2
Restricted Stock [Member]    
Restricted stock data for awards granted by the Company [Abstract]    
Awards granted (in shares) 387 368
Weighted average fair value of grant (in dollars per share) $ 51.35 $ 51.04
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