computation of earnings per share (basic and diluted) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share [Abstract]
Net income	$ 3,453	$ 4,356	$ 12,332	$ 9,137
Less: Undistributed earnings allocated to participating securities	(53)	(67)	(184)	(131)
Net income allocated to common stock	3,400	4,288	12,148	9,006
Weighted-average common shares issued	2,862	2,843	2,853	2,835
Less: Unvested restricted stock awards	(45)	(44)	(43)	(41)
Weighted average common shares outstanding used to calculate basic earnings per common share	2,817	2,799	2,810	2,794
Add: Dilutive effect of stock options	26	8	20	8
Weighted-average common shares outstanding used to calculate diluted earnings per common share	$ 2,843	$ 2,807	$ 2,830	$ 2,802
Earnings per common share (basic)	$ 1.21	$ 1.53	$ 4.32	$ 3.22
Earnings per common share (diluted)	$ 1.20	$ 1.53	$ 4.30	$ 3.21