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Note 7 - Common Stock Options - Changes in Common Stock Options Issued (Details) - $ / shares
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Options outstanding (in shares) 5,327,425 6,965,730 5,535,439
Granted (in shares) 809,500 230,500 2,563,500
Exercised (in shares) (1,348,374) (1,784,763) (771,376)
Forfeitures (in shares) (131,874) (84,042) (361,833)
Options outstanding (in shares) 4,656,677 5,327,425 6,965,730
Minimum [Member]      
Options outstanding, exercise price (in dollars per share) $ 2.71 $ 2.71 $ 2.50
Granted, exercise price (in dollars per share) 11.35 8.89 5.15
Exercised, exercise price (in dollars per share) 3.27 2.71 2.5
Forfeitures, exercise price (in dollars per share) 4.14 4.26 2.50
Options outstanding, exercise price (in dollars per share) 2.71 2.71 2.71
Maximum [Member]      
Options outstanding, exercise price (in dollars per share) 21.99 21.99 21.99
Granted, exercise price (in dollars per share) 20.20 21.80 19.87
Exercised, exercise price (in dollars per share) 15.00 19.09 15
Forfeitures, exercise price (in dollars per share) 21.99 21.80 16.50
Options outstanding, exercise price (in dollars per share) 21.74 21.99 21.99
Weighted Average [Member]      
Options outstanding, exercise price (in dollars per share) 6.09 6.09 7.30
Granted, exercise price (in dollars per share) 13.47 16.85 9.36
Exercised, exercise price (in dollars per share) 7.78 6.08 4.62
Forfeitures, exercise price (in dollars per share) 12.23 8.55 7.53
Options outstanding, exercise price (in dollars per share) $ 10.68 $ 6.09 $ 6.09