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Stockholders' equity - Warrants (Details) - USD ($)
$ / shares in Units, $ in Millions
 1 Months Ended 12 Months Ended
Jan. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Class of Warrant or Right [Line Items]      
New Common Shares Issued (number of shares)   2,500,285  
Warrant [Member]      
Class of Warrant or Right [Line Items]      
Class of Warrant or Right, Outstanding   1,745,868 0.00
Class of Warrant or Right, Exercise Price of Warrants or Rights   $ 3.25 $ 0.00
New Common Shares Issued (number of shares)   4,246,153  
Weighted Average Exercise Price Of Warrants Issued   $ 3.25  
Exercise of Warrants for Common Stock, shares   (2,500,285)  
Investment Warrants, Exercise Price   $ 2.12  
Class Of Warrant Of Right Exercisable   1,745,868  
Weighted Average Exercise Price of Warrants Exercisable   $ 3.25  
January 2018 Public Offering [Member] | Warrant [Member]      
Class of Warrant or Right [Line Items]      
Class of Warrant or Right, Exercise Price of Warrants or Rights   $ 3.25  
New Common Shares Issued (number of shares) 4,246,153 4,246,153  
Exercise of Warrants for Common Stock, shares   (249,824)  
January 2018 Public Offering, Discounted Warrants [Member] | January 2018 Public Offering [Member] | Warrant [Member]      
Class of Warrant or Right [Line Items]      
Class of Warrant or Right, Exercise Price of Warrants or Rights   $ 2.00  
New Common Shares Issued (number of shares)   2,250,461  
Stock and Warrants Issued During Period, Value, Preferred Stock and Warrants   $ 1.8