Note 3 - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2019 USD ($) $ / shares shares	Mar. 31, 2018 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Outstanding-Number of options (in shares) | shares	95,075	85,525
Granted-Number of options (in shares) | shares	24,700	25,000
Exercised-Number of options (in shares) | shares	(8,275)	(9,750)
Outstanding ending-Number of options (in shares) | shares	111,500	100,775
Exercisable-Number of options (in shares) | shares	37,350	27,275
Outstanding-Weighted average exercise price (in dollars per share) | $ / shares	$ 35.11	$ 30.94
Granted-Weighted average exercise price (in dollars per share) | $ / shares	38.15	43.60
Exercised-Weighted average exercise price (in dollars per share) | $ / shares	25.38	24.75
Outstanding ending-Weighted average exercise price (in dollars per share) | $ / shares	36.51	34.68
Exercisable-Weighted average exercise price (in dollars per share) | $ / shares	$ 29.96	$ 27.51
Outstanding ending-Weighted average remaining contractual term	3 years 29 days	3 years 2 months 23 days
Exercisable-Weighted average remaining contractual term	1 year 5 months 23 days	1 year 4 months 28 days
Outstanding ending-Aggregate intrinsic value | $	$ 270	$ 944
Exercisable-Aggregate intrinsic value | $	$ 270	$ 451