Note 3 - Stock Option Activity (Details) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2021 USD ($) $ / shares shares	Jun. 30, 2020 USD ($) $ / shares shares
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Outstanding-Number of options (in shares) | shares	116,550	103,350
Granted-Number of options (in shares) | shares	25,000	25,000
Exercised-Number of options (in shares) | shares	(19,025)	(9,550)
Forfeited-Number of options (in shares) | shares	(3,125)
Outstanding ending-Number of options (in shares) | shares	119,400	118,800
Exercisable-Number of options (in shares) | shares	45,475	44,600
Outstanding-Weighted average exercise price (in dollars per share) | $ / shares	$ 37.42	$ 36.96
Granted-Weighted average exercise price (in dollars per share) | $ / shares	32.50	36.50
Exercised-Weighted average exercise price (in dollars per share) | $ / shares	30.97	29.39
Forfeited-Weighted average exercise price (in dollars per share) | $ / shares	30.40
Outstanding ending-Weighted average exercise price (in dollars per share) | $ / shares	37.60	37.47
Exercisable-Weighted average exercise price (in dollars per share) | $ / shares	$ 4.73	$ 34.27
Outstanding ending-Weighted average remaining contractual term	2 years 2 months 1 day	2 years 8 months 15 days
Exercisable-Weighted average remaining contractual term	7 months 24 days	1 year 2 months 1 day
Outstanding ending-Aggregate intrinsic value | $	$ 0	$ 0
Exercisable-Aggregate intrinsic value | $	$ 0	$ 0