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Stock-based Payments - Stock Option Activity (Details) - USD ($)
$ / shares in Units, $ in Thousands
 12 Months Ended
Apr. 30, 2020
Apr. 30, 2019
Apr. 30, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]      
Shares, Outstanding, Beginning Balance 2,423,626 2,705,845  
Shares, Granted 229,833 206,790  
Shares, Exercised (248,495) (363,383)  
Shares, Canceled (11,824) (49,766)  
Shares, Forfeited (44,813) (9,750)  
Shares, Expired (76,669) (66,110)  
Shares, Outstanding, Ending Balance 2,271,658 2,423,626 2,705,845
Shares, Vested and expected to vest 2,271,658 2,423,626  
Shares, Vested 1,943,618 2,133,920  
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 $ 2.85  
Weighted Average Exercise Price, Granted (in usd per share) 5.33 9.86  
Weighted Average Exercise Price, Exercised (in usd per share) 2.31 2.22  
Weighted Average Exercise Price, Canceled (in usd per share) 7.96 3.20  
Weighted Average Exercise Price, Forfeited (in usd per share) 7.85 3.50  
Weighted Average Exercise Price, Expired (in usd per share) 8.04 15.10  
Weighted Average Exercise Price, Outstanding, Ending Balance (in usd per share) 3.23 3.19 $ 2.85
Weighted Average Exercise Price, Vested and Expected to Vest (in usd per share) 3.23 3.19  
Weighted Average Exercise Price, Vested (in usd per share) $ 2.83 $ 2.70  
Weighted Average Remaining Contractual Term, Outstanding, Beginning Balance (in years)   5 years 3 months 19 days 5 years 10 months 25 days
Weighted Average Remaining Contractual Life, Granted 8 years 1 month 20 days 7 years 10 months 25 days  
Weighted Average Remaining Contractual Term, Outstanding, Ending Balance (in years) 5 years 5 years 3 months 19 days  
Weighted Average Remaining Contractual Life (Years), Vested and expected to vest 5 years 5 years 3 months 19 days  
Weighted Average Remaining Contractual Life (Years), Vested 4 years 6 months 4 years 11 months  
Aggregate Intrinsic Value, Outstanding, Beginning Balance $ 14,557 $ 5,265  
Aggregate Intrinsic Value, Granted 544 77  
Aggregate Intrinsic Value, Outstanding, Ending Balance 10,663 14,557 $ 5,265
Aggregate Intrinsic Value, Vested and expected to vest 10,663 14,557  
Aggregate Intrinsic Value, Vested $ 9,898 $ 13,785  
Non- Employees      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]      
Shares, Outstanding, Beginning Balance 50,000 50,000  
Shares, Granted 0 0  
Shares, Exercised 0 0  
Shares, Canceled 0 0  
Shares, Forfeited 0 0  
Shares, Expired (6,668) 0  
Shares, Outstanding, Ending Balance 43,332 50,000 50,000
Shares, Vested and expected to vest 43,332 50,000  
Shares, Vested 17,501 18,335  
Directors and Employees      
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding [Roll Forward]      
Shares, Outstanding, Beginning Balance 2,373,626 2,655,845  
Shares, Granted 229,833 206,790  
Shares, Exercised (248,495) (363,383)  
Shares, Canceled (11,824) (49,766)  
Shares, Forfeited (44,813) (9,750)  
Shares, Expired (70,001) (66,110)  
Shares, Outstanding, Ending Balance 2,228,326 2,373,626 2,655,845
Shares, Vested and expected to vest 2,228,326 2,373,626  
Shares, Vested 1,926,117 2,115,585