Net Loss Per Share - Antidilutive Securities (Details) - shares
6 Months Ended
Jun. 30, 2019
Jun. 30, 2018
Diluted net loss per share attributable to common stockholders    
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding 17,468,405 67,001,348
Convertible preferred stock    
Diluted net loss per share attributable to common stockholders    
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding   51,845,438
Warrants to purchase convertible preferred stock    
Diluted net loss per share attributable to common stockholders    
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding   135,567
Restricted Common Stock    
Diluted net loss per share attributable to common stockholders    
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding 1,401,858 3,058,612
Stock options    
Diluted net loss per share attributable to common stockholders    
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding 16,066,547 11,961,731