Note 8 - Stockholders' Equity - Option Activity for Stock Option Plans (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Outstanding, options (in shares) | shares	3,023,639
Outstanding, weighted average exercise price, options (in dollars per share) | $ / shares	$ 1.40
Forfeited, options (in shares) | shares	(114,301)
Forfeited, weighted average exercise price, options (in dollars per share) | $ / shares	$ 1.11
Outstanding, options (in shares) | shares	2,909,338
Outstanding, weighted average exercise price, options (in dollars per share) | $ / shares	$ 1.41
Outstanding, weighted average remaining contractual life, options (Year)	9 years
Outstanding, aggregate intrinsic value, options | $
Vested and Expected to Vest, options (in shares) | shares	1,911,105
Vested and Expected to Vest, weighted average exercise price, options (in dollars per share) | $ / shares	$ 1.73
Vested and Expected to Vest, weighted average remaining contractual life, options (Year)	8 years 292 days
Vested and Expected to Vest, aggregate intrinsic value, options | $
Exercisable, options (in shares) | shares	876,955
Exercisable, weighted average exercise price, options (in dollars per share) | $ / shares	$ 2.79
Exercisable, weighted average remaining contractual life, options (Year)	7 years 328 days
Exercisable, aggregate intrinsic value, options | $