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Stock-Based Compensation (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended
Jul. 31, 2017
Jul. 31, 2016
Apr. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock-based compensation expense $ 945 $ 846  
Liability for payment of the RSTUs $ 28,606   $ 36,162
Employee Service-Based RSUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Awarded in period (in shares) 17,840    
Employee Performance-Based RSUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Awarded in period (in shares) 33,080    
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    
Non-Employee Service-Based Restricted Stock Units [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period, 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) 4,496    
Employee Service-Based RSTUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Restricted stock units non vested grants (in shares) 2,519    
RSTUs [Member]      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Vesting period, years 3 years    
Stock-based compensation expense $ 225 $ 51  
Liability for payment of the RSTUs $ 800   $ 1,500