Note 5 - Stock-based Compensation - Summary of Stock Options (Details)	6 Months Ended
	Jun. 30, 2019 USD ($) $ / shares shares
Shares outstanding (in shares) | shares	206,500
Shares outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 7.05
Shares outstanding, weighted average remaining contractual life (Year)	5 years
Shares outstanding, aggregate intrinsic value | $	$ 4,613,505
Shares vested and exercisable (in shares) | shares	156,000
Shares vested and exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 2.12
Shares vested and exercisable, weighted average remaining contractual life (Year)	3 years 182 days
Shares vested and exercisable, aggregate intrinsic value | $	$ 4,161,240