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Stockholder's equity (deficit) - Restricted stock activity narrative (Details) - shares
6 Months Ended
Jun. 30, 2019
Dec. 31, 2018
Number of Shares    
Unvested restricted common stock 539,827 80,190
Early exercise of unvested stock options 487,933  
Restricted stock | Founders [Member]    
Number of Shares    
Unvested restricted common stock 51,894