Net Loss Per Share - Antidilutive Securities (Details) - shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Diluted net loss per share attributable to common stockholders      
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding 17,015,005 55,007,222 34,211,280
Convertible preferred stock      
Diluted net loss per share attributable to common stockholders      
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding   43,933,006 29,570,662
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   135,567 133,333
Restricted Common Stock      
Diluted net loss per share attributable to common stockholders      
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding 2,230,235 6,277,014 348,120
Stock options      
Diluted net loss per share attributable to common stockholders      
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding 14,784,770 4,661,635 4,159,165
Series A redeemable convertible preferred stock      
Diluted net loss per share attributable to common stockholders      
Percentage of cumulative dividends per year 8.00%