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Stock-Based Compensation (Narrative) (Details) - USD ($)
$ in Thousands
3 Months Ended 9 Months Ended
Jan. 31, 2020
Jan. 31, 2019
Jan. 31, 2020
Jan. 31, 2019
Apr. 30, 2019
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Stock-based compensation expense $ 1,047 $ 668 $ 3,122 $ 2,290  
Liability for payment of the RSTUs 45,714   $ 45,714   $ 54,906
Employee Service-Based RSUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Awarded in period (shares)     42,691    
Vesting period     2 years    
Employee Performance-Based RSUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Awarded in period (shares)     61,379    
Common stock issuable per RSU granted (shares)     1    
Employee Performance-Based RSTUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted stock units non vested grants (shares)     6,483    
Employee Service-Based RSTUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Restricted stock units non vested grants (shares)     3,482    
RSTUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period, years     3 years    
Stock-based compensation expense 200 $ 100 $ 400 $ 200  
Liability for payment of the RSTUs $ 700   $ 700   $ 700
Cliff Vest [Member] | RSUs [Member]          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Vesting period, years     3 years