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Stock plans - Schedule of Activity Related to Restricted Stock (Details) - Restricted Stock - shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award      
Nonvested, January 1 21,309 12,448 12,362
Granted shares 17,345 11,468 10,628
Vested shares 10,120 2,607 10,542
Forfeited shares 1,069    
Nonvested, December 31 27,465 21,309 12,448