Note 13 - Stock Option Activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share Based Compensation Arrangement By Share Based Payment Award Options Outstanding Roll Forward
Outstanding-Number of options (in shares)	73,950	82,875	88,375
Exercised-Number of options (in shares)	(13,125)	(22,700)	(13,750)
Forfeited-Number of options (in shares)	(300)	(9,725)	(12,750)
Granted-Number of options (in shares)	25,000	23,500	21,000
Outstanding ending-Number of options (in shares)	85,525	73,950	82,875
Exercisable-Number of options (in shares)	19,175
Outstanding-Weighted average exercise price (in dollars per share)	$ 27.14	$ 24.33	$ 23.27
Exercised-Weighted average exercise price (in dollars per share)	22.42	20.90	18.11
Forfeited-Weighted average exercise price (in dollars per share)	22.58	25.60	31.80
Granted-Weighted average exercise price (in dollars per share)	37.60	30.40	29.25
Outstanding ending-Weighted average exercise price (in dollars per share)	30.94	$ 27.14	$ 24.33
Exercisable-Weighted average exercise price (in dollars per share)	$ 24.48
Outstanding ending-Weighted average remaining contractual term	2 years 7 months 28 days
Exercisable-Weighted average remaining contractual term	8 months 19 days
Outstanding ending-Aggregate intrinsic value	$ 1,151
Exercisable-Aggregate intrinsic value	$ 382