Stock plans - Schedule of Activity Related to Restricted Stock (Details) - 2017 Plan - shares	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Share-based Compensation Arrangement by Share-based Payment Award
Nonvested, January 1	39,470	36,281	31,923
Granted shares	23,428	19,787	19,818
Vested shares	16,791	16,403	15,460
Forfeited shares		195
(Surrendered) Forfeited shares	(12,524)
Nonvested, December 31	58,631	39,470	36,281