Net Loss Per Share - Antidilutive Securities (Details) - shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Diluted net loss per share attributable to common stockholders      
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding 16,570,124 15,480,421 17,015,005
Series A redeemable convertible preferred stock      
Diluted net loss per share attributable to common stockholders      
Percentage of cumulative dividends per year 8.00%    
Unvested restricted common stock      
Diluted net loss per share attributable to common stockholders      
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding 252,000 376,514 2,230,235
Stock options to purchase common stock      
Diluted net loss per share attributable to common stockholders      
Antidilutive securities excluded from computation of of diluted weighted average shares outstanding 16,318,124 15,103,907 14,784,770