Note 6 - Stock-based Compensation - Summary of Stock Options (Details)	9 Months Ended
	Sep. 30, 2021 USD ($) $ / shares shares
Shares outstanding (in shares) | shares	59,000
Shares outstanding, weighted average exercise price (in dollars per share) | $ / shares	$ 17.35
Shares outstanding, weighted average remaining contractual life (Year)	6 years 10 months 24 days
Shares outstanding, aggregate intrinsic value | $	$ 1,372,000
Shares vested and exercisable (in shares) | shares	49,000
Shares vested and exercisable, weighted average exercise price (in dollars per share) | $ / shares	$ 16.81
Shares vested and exercisable, weighted average remaining contractual life (Year)	6 years 9 months 18 days
Shares vested and exercisable, aggregate intrinsic value | $	$ 1,166,000