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Stock plans - Schedule of Activity Related to Restricted Stock (Details) - Restricted Stock - shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award    
Nonvested, January 1 12,362 14,309
Granted shares 2,362  
Vested shares 10,542 1,947
Nonvested, December 31 4,182 12,362