Note 2 - Computation of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2017		Sep. 30, 2017		Jun. 30, 2017		Mar. 31, 2017		Dec. 31, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Numerator for basic and diluted earnings per share - net income	$ 489		$ 2,554		$ 2,386		$ 2,860		$ 2,269		$ 2,292		$ 2,098		$ 2,265		$ 8,289	$ 8,924	$ 8,233
Denominator for basic earnings per share - weighted average shares outstanding																	3,428,970	3,386,766	3,337,505
Effect of dilutive securities - employee stock options																	16,841	9,073	13,034
Denominator for diluted earnings per share - adjusted weighted average shares outstanding																	3,445,811	3,395,839	3,350,539
Earnings per share - basic	$ 0.14	[1]	$ 0.74	[1]	$ 0.70	[1]	$ 0.84	[1]	$ 0.67	[1]	$ 0.68	[1]	$ 0.62	[1]	$ 0.67	[1]	$ 2.42	$ 2.64	$ 2.47
Earnings per share - diluted	$ 0.14	[1]	$ 0.74	[1]	$ 0.69	[1]	$ 0.83	[1]	$ 0.66	[1]	$ 0.67	[1]	$ 0.62	[1]	$ 0.67	[1]	$ 2.41	$ 2.63	$ 2.46

[1] Due to rounding, quarterly earnings per share may not sum to annual earnings per share