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Stock plans - Schedule of Activity Related to Restricted Stock (Details) - 2017 Plan - shares
12 Months Ended
Dec. 31, 2022
Dec. 31, 2021
Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award      
Nonvested, January 1 36,281 31,923 25,984
Granted shares 19,787 19,818 16,269
Vested shares 16,403 15,460 8,455
Forfeited shares 195 0 1,875
Nonvested, December 31 39,470 36,281 31,923