Note 3 - Stock-based Compensation - Stock Option Activity (Details)	9 Months Ended
	Sep. 30, 2020 $ / shares shares	Sep. 30, 2019 shares
Options outstanding (in 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)	0	370,000
Exercised (in shares)	(747,674)
Exercised, weighted average exercise price (in dollars per share) | $ / shares	$ 2.63
Canceled/expired/forfeited (in shares)
Canceled/expired/forfeited, weighted average exercise price (in dollars per share) | $ / shares
Options outstanding (in shares)	578,740
Options outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 3.43
Options exercisable (in shares)	172,167
Options exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 3.22
Non-Employee Directors [Member]
Granted to employees and non-employee directors (in shares)
Granted to employees and non-employee directors, weighted average exercise price (in dollars per share) | $ / shares