Note 6 - Stock-based Compensation - Stock Option Activity (Details)	6 Months Ended
	Jun. 30, 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	660,000
Granted, weighted-average exercise price (in dollars per share) | $ / shares	$ 1.02
Expired (in shares) | shares	(2,192)
Expired, weighted-average exercise price (in dollars per share) | $ / shares	$ 17.10
Options outstanding, number (in shares) | shares	3,213,797
Options outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 6.02
Options outstanding, weighted-average remaining contractual life (Year)	7 years 91 days
Exercisable (in shares) | shares	2,185,235
Exercisable, weighted-average exercise price (in dollars per share) | $ / shares	$ 7.46
Exercisable, weighted-average remaining contractual life (Year)	6 years 149 days