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Note 6 - Common Stock Options - Changes in Common Stock Options Issued (Details) - $ / shares
9 Months Ended 12 Months Ended
Sep. 30, 2020
Dec. 31, 2019
Options outstanding (in shares) 6,965,730  
Granted (in shares) 214,500  
Exercised (in shares) (1,704,905) (771,000)
Forfeitures (in shares) (57,042)  
Options outstanding (in shares) 5,418,283 6,965,730
Minimum [Member]    
Options outstanding, exercise price (in dollars per share) $ 2.71  
Granted, exercise price (in dollars per share) 8.89  
Exercised, exercise price (in dollars per share) 2.71  
Forfeitures, exercise price (in dollars per share) 4.26  
Options outstanding, exercise price (in dollars per share) 2.71 $ 2.71
Maximum [Member]    
Options outstanding, exercise price (in dollars per share) 21.99  
Granted, exercise price (in dollars per share) 14.77  
Exercised, exercise price (in dollars per share) 19.09  
Forfeitures, exercise price (in dollars per share) 21.80  
Options outstanding, exercise price (in dollars per share) 21.99 21.99
Weighted Average [Member]    
Options outstanding, exercise price (in dollars per share) 6.09  
Granted, exercise price (in dollars per share) 16.71  
Exercised, exercise price (in dollars per share) 6.11  
Forfeitures, exercise price (in dollars per share) 8.53  
Options outstanding, exercise price (in dollars per share) $ 6.20 $ 6.09