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Note 4 - Common Stock Options - Changes in Common Stock Options Issued (Details) - $ / shares
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2017
Mar. 31, 2017
Jun. 30, 2017
Sep. 30, 2017
Dec. 31, 2016
Options outstanding (in shares)       5,188,658  
Granted (in shares)       1,242,000  
Exercised (in shares) (18,000) (66,000) (205,000) (405,167) (204,000)
Exercised (in dollars per share) $ 2.50        
Forfeitures (in shares)       (474,451)  
Options outstanding (in shares) 5,551,040     5,551,040 5,188,658
Minimum [Member]          
Options outstanding, exercise price (in dollars per share)       $ 0.52  
Granted (in dollars per share)       4.24  
Exercised (in dollars per share)   $ 2.50 $ 0.93 0.93 $ 0.74
Forfeitures (in dollars per share)       0.94  
Options outstanding, exercise price (in dollars per share) $ 2.50     2.50 0.52
Maximum [Member]          
Options outstanding, exercise price (in dollars per share)       21.99  
Granted (in dollars per share)       12.62  
Exercised (in dollars per share)   $ 14.71 $ 10 14.71 19.09
Forfeitures (in dollars per share)       21.99  
Options outstanding, exercise price (in dollars per share) 21.99     21.99 21.99
Weighted Average [Member]          
Options outstanding, exercise price (in dollars per share)       8.49  
Granted (in dollars per share)       4.85  
Exercised (in dollars per share)       2.28  
Forfeitures (in dollars per share)       13.13  
Options outstanding, exercise price (in dollars per share) $ 7.73     $ 7.73 $ 8.49