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Stock plans - Schedule of Activity Related to Restricted Stock (Details) - Restricted Stock - shares
12 Months Ended
Dec. 31, 2016
Dec. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award    
Nonvested, January 1 14,309 17,245
Vested shares 1,947 2,936
Nonvested, December 31 12,362 14,309