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, 2020		Sep. 30, 2020		Jun. 30, 2020		Mar. 31, 2020		Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Numerator for basic and diluted earnings per share - net income	$ 4,151		$ 3,778		$ 3,934		$ 220		$ 2,745		$ 3,090		$ 3,143		$ 3,379		$ 12,083	$ 12,357	$ 11,335
Denominator for basic earnings per share - weighted average shares outstanding																	3,537,323	3,498,326	3,463,450
Effect of dilutive securities - employee stock options																	37	5,824	19,059
Denominator for diluted earnings per share - adjusted weighted average shares outstanding																	3,537,360	3,504,150	3,482,509
Earnings per share - basic	$ 1.17	[1]	$ 1.07	[1]	$ 1.11	[1]	$ 0.06	[1]	$ 0.78	[1]	$ 0.88	[1]	$ 0.90	[1]	$ 0.97	[1]	$ 3.42	$ 3.53	$ 3.27
Earnings per share - diluted	$ 1.17	[1]	$ 1.07	[1]	$ 1.11	[1]	$ 0.06	[1]	$ 0.78	[1]	$ 0.88	[1]	$ 0.90	[1]	$ 0.97	[1]	$ 3.42	$ 3.53	$ 3.25

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