Note 12 - Stock-based Compensation - Option Activity (Details) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) $ / shares shares
Outstanding (in shares) | shares	11,422,705
Outstanding (in dollars per share) | $ / shares	$ 3.64
Granted (in shares) | shares	2,058,128
Granted, weighted-average exercise price (in dollars per share) | $ / shares	$ 2.53
Forfeited (in shares) | shares	(442,979)
Forfeited, weighted-average exercise price (in dollars per share) | $ / shares	$ 2.57
Expired (in shares) | shares	(277,413)
Expired, weighted-average exercise price (in dollars per share) | $ / shares	$ 5.31
Exercised (in shares) | shares	(111,702)
Exercised, weighted-average exercise price (in dollars per share) | $ / shares	$ 2.41
Outstanding (in shares) | shares	12,648,739
Outstanding, weighted-average exercise price (in dollars per share) | $ / shares	$ 3.47
Outstanding, weighted-average remaining contractual life (Year)	6 years 182 days
Outstanding, aggregate intrinsic value | $	$ 197
Vested and exercisable options (in shares) | shares	8,583,475
Vested and exercisable options, weighted-average exercise price (in dollars per share) | $ / shares	$ 3.90
Vested and exercisable options, weighted-average remaining contractual life (Year)	5 years 219 days
Vested and exercisable options, aggregate intrinsic value | $	$ 90
Vested and expected to vest (in shares) | shares	12,648,739
Vested and expected to vest, weighted-average exercise price (in dollars per share) | $ / shares	$ 3.47
Vested and expected to vest, weighted-average remaining contractual life (Year)	6 years 182 days
Vested and expected to vest, aggregate intrinsic value | $	$ 197