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Note 10 - Stock Compensation Plans - Stock Option Activity (Details) - $ / shares
 12 Months Ended 216 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2020
Options outstanding, beginning of year (in shares) 126,500 207,500  
Options cancelled (in shares) (16,000) (282,680)
Options exercised (in shares) (107,000) (947,320)
Options granted (in shares) 42,000 1,356,500
Options outstanding, end of year (in shares) 126,500 126,500 126,500
Options available for grant (in shares) 885,620 763,000 885,620
Options exercisable (in shares) 102,500 84,500 102,500
Granted (in dollars per share) $ 25.45  
Exercised (in dollars per share) 1.97  
Outstanding (in dollars per share) 8.94 8.94 $ 8.94
Exercisable (in dollars per share) 5.61 2.21 5.61
Minimum [Member]      
Granted (in dollars per share) 19.99  
Exercised (in dollars per share) 0.69  
Outstanding (in dollars per share) 1.52 1.52 1.52
Maximum [Member]      
Granted (in dollars per share)   39.11  
Exercised (in dollars per share)   7.80  
Outstanding (in dollars per share) $ 39.11 $ 39.11 $ 39.11