Note 3 - Stock-based Compensation - Stock Option Activity (Details) - $ / shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Options outstanding (in shares)	1,022,913
Options outstanding, weighted average exercise price (in dollars per share)	$ 2.69
Granted to employees and non-employee directors (in shares)	310,000	289,750
Granted to employees and non-employee directors, weighted average exercise price (in dollars per share)	$ 3.62
Exercised (in shares)	(33,334)
Exercised, weighted average exercise price (in dollars per share)	$ 1.80
Canceled/expired/forfeited (in shares)
Canceled/expired/forfeited, weighted average exercise price (in dollars per share)	$ 0
Options outstanding (in shares)	1,299,579
Options outstanding, weighted average exercise price (in dollars per share)	$ 2.93
Options exercisable (in shares)	461,751
Options exercisable, weighted average exercise price (in dollars per share)	$ 2.50