Note 7 - Stockholders' Equity - Option Activity for Stock Option Plans (Details)	3 Months Ended
	Mar. 31, 2018 USD ($) $ / shares shares
Outstanding, options (in shares) | shares	1,156,027
Outstanding, weighted average exercise price, options (in dollars per share) | $ / shares	$ 3.92
Granted, options (in shares) | shares	80,000
Granted, weighted average exercise price, options (in dollars per share) | $ / shares	$ 3
Forfeited (in shares) | shares	(2,435)
Forfeited (in dollars per share) | $ / shares	$ 3.53
Expired, options (in shares) | shares	(27,182)
Expired, weighted average exercise price, options (in dollars per share) | $ / shares	$ 4.67
Outstanding, options (in shares) | shares	1,206,410
Outstanding, weighted average exercise price, options (in dollars per share) | $ / shares	$ 3.84
Outstanding, weighted average remaining contractual life, options (Year)	8 years 182 days
Outstanding, aggregate intrinsic value, options | $
Vested and Expected to Vest, options (in shares) | shares	943,103
Vested and Expected to Vest, weighted average exercise price, options (in dollars per share) | $ / shares	$ 4.07
Vested and Expected to Vest, weighted average remaining contractual life, options (Year)	8 years 109 days
Vested and Expected to Vest, aggregate intrinsic value, options | $
Exercisable, options (in shares) | shares	306,582
Exercisable, weighted average exercise price, options (in dollars per share) | $ / shares	$ 6.22
Exercisable, weighted average remaining contractual life, options (Year)	5 years 255 days
Exercisable, aggregate intrinsic value, options | $