Note 3 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Options outstanding (in shares)	1,326,414
Options outstanding, weighted average exercise price (in dollars per share)	$ 2.86
Granted to employees and non-employee directors (in shares)	0	310,000
Granted to employees and non-employee directors, weighted average exercise price (in dollars per share)	$ 0
Exercised (in shares)	(712,839)
Exercised, weighted average exercise price (in dollars per share)	$ 2.58
Canceled/expired/forfeited (in shares)
Canceled/expired/forfeited, weighted average exercise price (in dollars per share)	$ 0
Options outstanding (in shares)	613,575
Options outstanding, weighted average exercise price (in dollars per share)	$ 3.43
Options exercisable (in shares)	182,002
Options exercisable, weighted average exercise price (in dollars per share)	$ 3.24