Note 3 - Stock-based Compensation - Stock Option Activity (Details)	6 Months Ended
	Jun. 30, 2020 $ / shares shares	Jun. 30, 2019 shares
Options outstanding (in shares) | shares	1,326,414
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 2.86
Granted to employees and non-employee directors (in shares) | shares	0	310,000
Granted to employees and non-employee directors, weighted average exercise price (in dollars per share) | $ / shares
Exercised (in shares) | shares	(737,674)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 2.61
Canceled/expired/forfeited (in shares) | shares
Canceled/expired/forfeited, weighted average exercise price (in dollars per share) | $ / shares
Options outstanding (in shares) | shares	588,740
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 3.33
Options exercisable (in shares) | shares	157,167
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 2.76