Note 9 - Stock-based Compensation - Stock Option Activity (Details)	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Balance (in shares) | shares	2,207,409
Balance (in dollars per share) | $ / shares	$ 8.09
Granted (in shares) | shares	340,041
Granted (in dollars per share) | $ / shares	$ 2.49
Expired (in shares) | shares	(1,461)
Expired (in dollars per share) | $ / shares	$ 15
Options outstanding (in shares) | shares	2,545,989
Options outstanding, weighted-average per share exercise price (in dollars per share) | $ / shares	$ 7.34
Options outstanding, weighted-average remaining contractual life (Year)	7 years 135 days
Exercisable (in shares) | shares	1,746,809
Exercisable (in dollars per share) | $ / shares	$ 8.51
Exercisable (Year)	6 years 226 days