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Share-Based Payments - Summary of Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
6 Months Ended 12 Months Ended
Oct. 31, 2019
Apr. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]    
Shares, Outstanding, Beginning Balance 2,423,626  
Shares, Granted 154,000  
Shares, Exercised (625)  
Shares, Forfeited (41,875)  
Shares, Canceled (4,583)  
Shares, Expired (76,669)  
Shares, Outstanding, Ending Balance 2,453,874  
Shares, Vested and expected to vest 2,453,874  
Shares, Exercisable 2,085,938  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Weighted Average Exercise Price [Abstract]    
Weighted Average Exercise Price, Outstanding, Beginning Balance (in usd per share) $ 3.19  
Weighted Average Exercise Price, Granted (in usd per share) 5.23  
Weighted Average Exercise Price, Exercised (in usd per share) 5.81  
Weighted Average Exercise Price, Forfeited (in usd per share) 7.99  
Weighted Average Exercise Price, Canceled (in usd per share) 11.90  
Weighted Average Exercise Price, Expired (in usd per share) 8.04  
Weighted Average Exercise Price, Outstanding, Ending Balance (in usd per share) 3.07  
Weighted Average Exercise Price, Vested and expected to vest (in usd per share) 3.07  
Weighted Average Exercise Price, Exercisable (in usd per share) $ 2.64  
Weighted Average Remaining Contractual Term, Outstanding, Beginning Balance (in years)   5 years 3 months 19 days
Weighted Average Remaining Contractual Term, Outstanding, Ending Balance (in years) 5 years 1 month 6 days  
Weighted Average Remaining Contractual Life, Vested and expected to vest (in years) 5 years 1 month 6 days  
Weighted Average Remaining Contractual Life, Exercisable (in years) 4 years 4 months 24 days  
Aggregate Intrinsic Value, Outstanding, Beginning Balance $ 14,557  
Aggregate Intrinsic Value, Outstanding, Ending Balance 6,216  
Aggregate Intrinsic Value, Vested and expected to vest 6,216  
Aggregate Intrinsic Value, Exercisable $ 6,004  
Non-employees    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]    
Shares, Outstanding, Beginning Balance 50,000  
Shares, Granted 0  
Shares, Exercised 0  
Shares, Forfeited 0  
Shares, Canceled 0  
Shares, Expired (6,668)  
Shares, Outstanding, Ending Balance 43,332  
Shares, Vested and expected to vest 43,332  
Shares, Exercisable 17,501  
Directors and employees    
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]    
Shares, Outstanding, Beginning Balance 2,373,626  
Shares, Granted 154,000  
Shares, Exercised (625)  
Shares, Forfeited (41,875)  
Shares, Canceled (4,583)  
Shares, Expired (70,001)  
Shares, Outstanding, Ending Balance 2,410,542  
Shares, Vested and expected to vest 2,410,542  
Shares, Exercisable 2,068,437