calculation of earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Earnings Per Share [Abstract]
Net income applicable to common shareholders	$ 11,940	$ 11,136	$ 8,824
Less: Undistributed earnings allocated to participating securities	(165)	(160)	(111)
Net income allocated to common stock	11,775	10,976	8,713
Weighted average common shares issued	2,837	2,817	2,798
Less: Unvested restricted stock awards	(39)	(35)	(35)
Weighted average common shares outstanding used to calculate basic earnings per common share	2,798	2,782	2,763
Add: Dilutive effect of stock options	8	12	17
Weighted average common shares outstanding used to calculate diluted earnings per common share	$ 2,806	$ 2,794	$ 2,780
Earnings per common share (basic)	$ 4.21	$ 3.95	$ 3.15
Earnings per common share (diluted)	$ 4.20	$ 3.93	$ 3.13