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Disclosure of Compensation Related Costs, Share Based Payments (Details) - 2009 Stock Incentive Plan - USD ($)
3 Months Ended
Jul. 31, 2018
Jul. 31, 2017
Apr. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award      
Number of shares available for grant (in shares) 2,657,030    
Allocated share-based compensation expense $ 10,154,000 $ 9,807,000  
Employee Stock Option      
Share-based Compensation Arrangement by Share-based Payment Award      
Number of options outstanding (in shares) 178,073   181,673
Aggregate intrinsic value for outstanding options $ 12,452,000    
Weighted average remaining contractual life (in years) 2 years 4 months 24 days    
Aggregate intrinsic value for exercised options $ 225,000    
Unrecognized compensation costs related to plan 0    
Restricted Stock Units      
Share-based Compensation Arrangement by Share-based Payment Award      
Unrecognized compensation costs related to plan $ 14,079,000    
Return on invested capital measurement period 3 years