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Stock plans - Schedule of Activity Related to Restricted Stock (Details) - Restricted Stock - shares
12 Months Ended
Dec. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Share-based Compensation Arrangement by Share-based Payment Award      
Nonvested, January 1 31,923 25,984 12,892
Granted shares 19,818 16,269 17,345
Vested shares 15,460 8,455 2,542
Forfeited shares   1,875 1,711
Nonvested, December 31 36,281 31,923 25,984