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Stock-Based Compensation (Narrative) (Details) - USD ($)
$ / shares in Units, $ in Thousands
3 Months Ended 9 Months Ended
Jan. 31, 2016
Jan. 31, 2015
Jan. 31, 2016
Jan. 31, 2015
Apr. 30, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Granted non-statutory stock options to key employees (in shares)     30,700    
Non-statutory stock options, weighted average exercise price (in usd per share)     $ 57.11    
Vesting period, years     3 years    
Contractual terms, years     10 years    
Stock-based compensation expense $ 856 $ 773 $ 2,651 $ 2,537  
Liability for payment of the RSTUs $ 33,920   $ 33,920   $ 30,120
Employee Service-Based RSUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period, years 2 years        
Employee Performance-Based RSUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Awarded in period (in shares)     48,201    
Common stock issuable per RSU granted (in shares)     1    
Employee and Non Employee Director Service-Based RSUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Awarded in period (in shares)     22,349    
Common stock issuable per RSU granted (in shares)     1    
RSUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period, years     3 years    
Employee Performance-Based RSTUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted stock units non vested grants (in shares)     7,616    
Employee Service-Based RSTUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted stock units non vested grants (in shares)     2,499    
RSTUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period, years     3 years    
Stock-based compensation expense $ 94 $ 61 $ 595 $ 225  
Liability for payment of the RSTUs $ 1,015   $ 1,015   $ 420