Note 3 - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2020 USD ($) $ / shares shares	Sep. 30, 2019 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Outstanding-Number of options (in shares) | shares	103,350	95,075
Granted-Number of options (in shares) | shares	25,000	24,700
Exercised-Number of options (in shares) | shares	(9,550)	(8,775)
Forfeited-Number of options (in shares) | shares		(1,500)
Outstanding ending-Number of options (in shares) | shares	118,800	109,500
Exercisable-Number of options (in shares) | shares	44,600	36,350
Outstanding-Weighted average exercise price (in dollars per share) | $ / shares	$ 36.96	$ 35.11
Granted-Weighted average exercise price (in dollars per share) | $ / shares	36.50	38.15
Exercised-Weighted average exercise price (in dollars per share) | $ / shares	29.39	25.64
Forfeited-Weighted average exercise price (in dollars per share) | $ / shares		37.20
Outstanding ending-Weighted average exercise price (in dollars per share) | $ / shares	37.47	36.53
Exercisable-Weighted average exercise price (in dollars per share) | $ / shares	$ 34.27	$ 29.95
Outstanding ending-Weighted average remaining contractual term	2 years 5 months 15 days	2 years 4 months 2 days
Exercisable-Weighted average remaining contractual term	11 months 1 day	8 months 19 days
Outstanding ending-Aggregate intrinsic value | $	$ 0	$ 357
Exercisable-Aggregate intrinsic value | $	$ 0	$ 318