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Stock-based compensation - Summary of stock option activity (Details) - USD ($)
 3 Months Ended 12 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Shares    
Shares outstanding, Beginning Balance 2,156,250  
Shares granted 587,500  
Shares exercised 0  
Shares canceled or forfeited (71,681)  
Shares outstanding, Ending Balance 2,672,069 2,156,250
Shares exercisable 1,128,717  
Shares vested or expected to vest 2,672,069  
Weighted-Average Exercise Price    
Shares outstanding, weighted-average exercise price, Beginning Balance (in usd per share) $ 8.66  
Shares granted, weighted-average exercise price (in usd per share) 4.58  
Shares exercised, weighted-average exercise price (in usd per share) 0.00  
Shares canceled or forfeited, weighted-average exercise price (in usd per share) 11.93  
Shares outstanding, weighted-average exercise price, Ending Balance (in usd per share) 7.68 $ 8.66
Shares exercisable, weighted-average exercise price (in usd per share) 7.74  
Shares vested or expected to vest, weighted-average exercise price (in usd per share) $ 7.68  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Additional Disclosures [Abstract]    
Outstanding, weighted-average remaining contractual term (in years) 7 years 10 months 6 days 7 years 11 months 9 days
Exercisable, weighted-average remaining contractual term (in years) 6 years 8 months 27 days  
Vested or expected to vest, weighted-average remaining contractual term (in years) 7 years 10 months 6 days  
Outstanding, aggregate intrinsic value $ 1,031,433 $ 1,605,684
Exercisable, aggregate intrinsic value 740,304  
Vested or expected to vest, aggregate intrinsic value $ 1,031,433