Note 3 - Stock-based Compensation - Stock Option Activity (Details)	9 Months Ended
	Sep. 30, 2019 $ / shares shares	Sep. 30, 2018 shares
Options outstanding (in shares) | shares	1,022,913
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 2.69
Granted to employees and non-employee directors (in shares) | shares	370,000	349,750
Granted to employees and non-employee directors, weighted average exercise price (in dollars per share) | $ / shares	$ 3.59
Exercised (in shares) | shares	(49,833)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 1.90
Canceled/expired/forfeited (in shares) | shares
Canceled/expired/forfeited, weighted average exercise price (in dollars per share) | $ / shares	$ 0
Options outstanding (in shares) | shares	1,343,080
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 2.96
Options exercisable (in shares) | shares	676,580
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 2.44