Note 8 - Stock-based Compensation - Stock Option Activity (Details)	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Balance at January 1, 2017 (in shares) | shares	2,207,409
Balance at January 1, 2017 (in dollars per share) | $ / shares	$ 8.09
Granted (in shares) | shares	320,041
Granted (in dollars per share) | $ / shares	$ 2.54
Expired (in shares) | shares	(1,461)
Expired (in dollars per share) | $ / shares	$ 15
Options outstanding, weighted-average per share exercise price (in dollars per share) | $ / shares	$ 7.38
Options outstanding, weighted-average remaining contractual life (Year)	7 years 219 days
Exercisable at June 30, 2017 (in shares) | shares	1,636,919
Exercisable at June 30, 2017 (in dollars per share) | $ / shares	$ 8.80
Exercisable at June 30, 2017 (Year)	6 years 270 days
Options outstanding (in shares) | shares	2,525,989