Summary of significant accounting policies - Earnings per share (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Basic earnings per share
Net income, Basic	$ 27,380	$ 38,090	$ 43,519
Average common shares outstanding - Basic	7,107,908	7,168,092	7,196,160
Earnings per share - Basic	$ 3.85	$ 5.31	$ 6.05
Diluted earnings per share
Net income, Diluted	$ 27,380	$ 38,090	$ 43,519
Average common shares outstanding - Diluted	7,151,471	7,211,643	7,235,303
Earnings per share - Diluted	$ 3.83	$ 5.28	$ 6.02