Note 3 - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) $ / shares shares	Mar. 31, 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)	(8,275)
Outstanding ending-Number of options (in shares) | shares	118,800	111,500
Exercisable-Number of options (in shares) | shares	44,600	37,350
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	25.38
Outstanding ending-Weighted average exercise price (in dollars per share) | $ / shares	37.47	36.51
Exercisable-Weighted average exercise price (in dollars per share) | $ / shares	$ 34.27	$ 29.96
Outstanding ending-Weighted average remaining contractual term	2 years 11 months 15 days	3 years 29 days
Exercisable-Weighted average remaining contractual term	1 year 5 months 1 day	1 year 5 months 23 days
Outstanding ending-Aggregate intrinsic value | $	$ 0	$ 270
Exercisable-Aggregate intrinsic value | $	$ 0	$ 270