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Treasury stock (Details) - $ / shares
Oct. 16, 2017
Oct. 17, 2016
Oct. 16, 2015
Feb. 19, 2015
Dec. 31, 2018
Dec. 31, 2017
Treasury stock            
Shares of treasury stock held         33,801 33,801
Cancellation of treasury stock       55,905    
Original purchase price of treasury stock (in dollars per share)       $ 0.002    
Restricted stock            
Treasury stock            
Shares surrendered by holder to cover taxes associated with vesting 14,895 9,709 9,197