Note 3 - Stocked-based Compensation - Stock Option Activity (Details)	3 Months Ended
	Mar. 31, 2021 $ / shares shares
Options outstanding (in shares) | shares	568,740
Options outstanding (in dollars per share) | $ / shares	$ 3.42
Granted to employees and non-employee directors (in shares) | shares
Granted to employees and non-employee directors (in dollars per share) | $ / shares
Exercised (in shares) | shares	(89,494)
Exercised (in dollars per share) | $ / shares	$ 3.45
Canceled/expired/forfeited (in shares) | shares
Canceled/expired/forfeited (in dollars per share) | $ / shares
Options outstanding, March 31, 2021 (in shares) | shares	479,246
Options outstanding, March 31, 2021 (in dollars per share) | $ / shares	$ 3.42
Options exercisable, March 31, 2021 (in shares) | shares	182,002
Options exercisable, March 31, 2021 (in dollars per share) | $ / shares	$ 5.80