Quarterly report pursuant to Section 13 or 15(d)

Stockholders' Equity - Summary of Stock Option Activity (Details)

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Stockholders' Equity - Summary of Stock Option Activity (Details) - USD ($)
6 Months Ended
Oct. 31, 2019
Apr. 30, 2019
Oct. 31, 2019
Number of Shares      
Exercisable, October 31, 2019 (in shares) 1,902,579   1,902,579
Stock Incentive Plan and Stock Option Grants to Employees and Directors      
Number of Shares      
Balance Outstanding, April 30, 2019 (in shares)     3,408,154
Options granted (in shares)     89,000
Exercised (in shares)     (453,660)
Forfeited (in shares)     (22,363)
Balance Outstanding, October 31, 2019 (in shares) 3,021,131 3,408,154 3,021,131
Exercisable, October 31, 2019 (in shares) 1,902,579   1,902,579
Weighted Average Exercise Price      
Balance Outstanding, April 30, 2019 (in dollars per share)     $ 4.44
Granted (in dollars per share)     4.03
Exercised (in dollars per share)     2.20
Forfeited (in dollars per share)     5.92
Balance Outstanding, October 31, 2019 (in dollars per share) $ 4.57 $ 4.44 4.57
Exercisable, October 31, 2019 (in dollars per share) $ 3.78   $ 3.78
Stock Option Activity, Additional Disclosures      
Options outstanding, weighted average remaining contractual term 2 years 10 months 24 days 2 years 10 months 24 days  
Options outstanding, aggregate intrinsic value $ 6,880,644 $ 6,880,644 $ 6,880,644
Options exercisable, weighted average remaining contractual term 1 year 10 months 9 days    
Options exercisable, aggregate intrinsic value $ 7,545,471   $ 7,545,471