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Stock-Based Compensation (Narrative) (Details)
$ in Thousands
12 Months Ended
Apr. 30, 2016
USD ($)
plan
award
shares
Apr. 30, 2015
USD ($)
Apr. 30, 2014
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of stock-based compensation plans | award 2    
Stock-based compensation expense, before income taxes $ 3,609 $ 3,497 $ 3,295
Number of stock incentive plans | plan 4    
Shares available for awards | shares 935,626    
Contractual terms, years 1 year 8 months 12 days    
Vesting period, years 3 years    
Total fair value of options vested $ 700 700 700
Total unrecognized compensation expense related to unvested stock options granted $ 600    
Expected to be recognized over a weighted average period 1 year 8 months 12 days    
Cash received from options exercised $ 8,100 14,300 14,700
Tax benefit realized from the tax deduction from option exercises 4,300 2,800 2,000
Accrued compensation and related expenses $ 35,389 30,120  
Non Employee Directors [Member] | Minimum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Contractual terms, years 4 years    
Non Employee Directors [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Contractual terms, years 10 years    
Employees [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Contractual terms, years 10 years    
Stock Options [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period, years 3 years    
Stock Options [Member] | Maximum [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Contractual terms, years 10 years    
Restricted Stock Units (RSUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period, years 3 years    
Total unrecognized compensation expense related to unvested stock options granted $ 3,300    
Expected to be recognized over a weighted average period 1 year 8 months    
Employee And Non Employee Director Service Based Restricted Stock Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period, years 2 years    
Performance-Based Restricted Stock Tracking Units (RSTUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares approved for grants | shares 7,616    
Service-Based Restricted Stock Tracking Units (RSTUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Shares approved for grants | shares 2,499    
Restricted Stock Tracking Units (RSTUs) [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation expense, before income taxes $ 800 400 $ 100
Vesting period, years 3 years    
Accrued compensation and related expenses $ 1,200 $ 400