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Disclosure of Compensation Related Costs, Share Based Payments (Details) - USD ($)
6 Months Ended
Oct. 31, 2019
Oct. 31, 2018
Apr. 30, 2019
Employee Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of shares available for grant reduction per stock option issued (in shares) 1    
Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of shares available for grant reduction per equity instruments other options issued (in shares) 2    
2009 Stock Incentive Plan      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of shares available for grant (in shares) 2,635,034    
Allocated share-based compensation expense $ 9,922,000 $ 12,151,000  
2009 Stock Incentive Plan | Employee Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of options outstanding (in shares) 57,866   109,827
Aggregate intrinsic value for outstanding options $ 7,315,000    
Weighted average remaining contractual life (in years) 1 year 8 months 1 day    
Aggregate intrinsic value for exercised options $ 5,643,000    
Unrecognized compensation costs related to plan 0    
2009 Stock Incentive Plan | Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award      
Unrecognized compensation costs related to plan $ 18,981,000    
Return on invested capital measurement period 3 years