Note 3 - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Outstanding-Number of options (in shares)	85,525	73,950
Granted-Number of options (in shares)	25,000	25,000
Exercised-Number of options (in shares)	(10,000)	(10,675)
Forfeited-Number of options (in shares)	(1,600)	(100)
Outstanding ending-Number of options (in shares)	98,925	88,175
Exercisable-Number of options (in shares)	26,525	21,825
Outstanding-Weighted average exercise price (in dollars per share)	$ 30.94	$ 27.14
Granted-Weighted average exercise price (in dollars per share)	43.60	37.60
Exercised-Weighted average exercise price (in dollars per share)	24.86	22.24
Forfeited-Weighted average exercise price (in dollars per share)	32.52	21.35
Outstanding ending-Weighted average exercise price (in dollars per share)	34.73	30.71
Exercisable-Weighted average exercise price (in dollars per share)	$ 27.46	$ 24.33
Outstanding ending-Weighted average remaining contractual term	2 years 11 months 26 days	3 years 1 month 2 days
Exercisable-Weighted average remaining contractual term	1 year 1 month 20 days	1 year 1 month 24 days
Outstanding ending-Aggregate intrinsic value	$ 1,180	$ 849
Exercisable-Aggregate intrinsic value	$ 509	$ 349