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Common stock - Narrative (Details) - USD ($)
1 Months Ended 9 Months Ended 12 Months Ended
Apr. 30, 2014
Mar. 31, 2014
Sep. 30, 2017
Dec. 31, 2016
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Common stock, shares authorized (in shares)     100,000,000 100,000,000
Common stock, par value (in usd per share)     $ 0.0001 $ 0.0001
Restricted stock        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock, shares sold (in shares) 867,314 4,553,415    
Restricted common stock, price per share (in usd per share)   $ 0.0004    
Proceeds from sale of restricted common stock to founders   $ 1,950    
Restricted common stock, repurchase period (in days)     90 days  
Restricted common stock, repurchase price (in usd per share)     $ 0.0004  
Restricted common stock, shares outstanding (in shares)     4,996,999  
Restricted stock | Non-employee Consultants and Advisers        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock, shares sold (in shares)       18,194
Restricted common stock, repurchase period (in days)     90 days  
Restricted common stock, repurchase price (in usd per share)     $ 0.0001  
Restricted common stock, shares outstanding (in shares)     11,485  
Restricted stock | Vests upon issuance        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock, percent vested     25.00%  
Restricted stock | Vests ratably over four years        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Restricted common stock, percent vested     75.00%  
Restricted common stock, vesting period (in years)     4 years