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Note 6 - Stock-based Compensation - Stock Option Activity (Details)
 3 Months Ended
Mar. 31, 2018
$ / shares
shares
Options outstanding, number (in shares) | shares 2,555,989
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares $ 7.32
Granted (in shares) | shares 500,000
Granted, weighted-average exercise price (in dollars per share) | $ / shares $ 1.18
Expired (in shares) | shares (2,192)
Expired, weighted-average exercise price (in dollars per share) | $ / shares $ 17.10
Options outstanding, number (in shares) | shares 3,053,797
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares $ 6.30
Options outstanding, weighted-average remaining contractual life (Year) 7 years 131 days
Exercisable (in shares) | shares 2,011,881
Exercisable, weighted-average exercise price (in dollars per share) | $ / shares $ 7.84
Exercisable, weighted-average remaining contractual life (Year) 6 years 167 days