Note 8 - Stockholders' Equity - Option Activity for Stock Option Plans (Details)	6 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares
Outstanding, options (in shares) | shares	302,368
Outstanding, weighted average exercise price, options (in dollars per share) | $ / shares	$ 13.99
Forfeited, options (in shares) | shares	(16,139)
Forfeited, weighted average exercise price, options (in dollars per share) | $ / shares	$ 11.71
Outstanding, options (in shares) | shares	286,229
Outstanding, weighted average exercise price, options (in dollars per share) | $ / shares	$ 14.12
Outstanding, weighted average remaining contractual life, options (Year)	9 years
Outstanding, aggregate intrinsic value, options | $
Vested and Expected to Vest, options (in shares) | shares	196,219
Vested and Expected to Vest, weighted average exercise price, options (in dollars per share) | $ / shares	$ 16.92
Vested and Expected to Vest, weighted average remaining contractual life, options (Year)	8 years 219 days
Vested and Expected to Vest, aggregate intrinsic value, options | $
Exercisable, options (in shares) | shares	84,804
Exercisable, weighted average exercise price, options (in dollars per share) | $ / shares	$ 28.17
Exercisable, weighted average remaining contractual life, options (Year)	7 years 255 days
Exercisable, aggregate intrinsic value, options | $