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Stock-Based Compensation (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Millions
3 Months Ended 6 Months Ended
Oct. 31, 2017
Oct. 31, 2016
Oct. 31, 2017
Oct. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation (benefit) expense $ 4.0 $ 4.6 $ 2.5 $ 4.9
Restricted Stock [Member]        
Restricted stock data for awards granted by the Company [Abstract]        
Awards granted (in shares)     405 509
Weighted average fair value of grant (in dollars per share) $ 51.44 $ 50.56 $ 51.44 $ 50.56
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