Note 4 - Stock-based Compensation - Summary of Stock Options (Details)	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares
Shares outstanding (in shares) | shares	194,500
Shares outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 5.07
Shares outstanding, weighted average remaining contractual life (Year)	4 years 328 days
Shares outstanding, aggregate intrinsic value | $	$ 5,226,180
Shares vested and exercisable (in shares) | shares	146,000
Shares vested and exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 1.83
Shares vested and exercisable, weighted average remaining contractual life (Year)	3 years 146 days
Shares vested and exercisable, aggregate intrinsic value | $	$ 4,395,480