Note 3 - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Outstanding-Number of options (in shares)	73,950	82,875
Granted-Number of options (in shares)	25,000	23,500
Exercised-Number of options (in shares)	(10,375)	(17,900)
Forfeited-Number of options (in shares)	(100)	(9,225)
Outstanding ending-Number of options (in shares)	88,475	79,250
Exercisable-Number of options (in shares)	22,125	24,125
Outstanding-Weighted average exercise price (in dollars per share)	$ 27.14	$ 24.33
Granted-Weighted average exercise price (in dollars per share)	37.60	30.40
Exercised-Weighted average exercise price (in dollars per share)	22.21	20.65
Forfeited-Weighted average exercise price (in dollars per share)	21.35	25.77
Outstanding ending-Weighted average exercise price (in dollars per share)	30.68	26.79
Exercisable-Weighted average exercise price (in dollars per share)	$ 24.31	$ 22.38
Outstanding ending-Weighted average remaining contractual term	3 years 3 months 29 days	3 years 2 months 19 days
Exercisable-Weighted average remaining contractual term	1 year 4 months 21 days	1 year 4 months 13 days
Outstanding ending-Aggregate intrinsic value	$ 661	$ 264
Exercisable-Aggregate intrinsic value	$ 306	$ 184