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Net Loss Per Share Attributable to Common Stockholders - Anti-dilutive effect (Details) - shares
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 30, 2019
Sep. 30, 2019
Dec. 31, 2018
Dec. 31, 2017
Antidilutive Securities        
Antidilutive securities excluded from computation of diluted net loss per share     15,802,648 5,043,746
Preferred Stock        
Antidilutive Securities        
Antidilutive securities excluded from computation of diluted net loss per share     15,560,569 4,978,957
Option to purchase common stock        
Antidilutive Securities        
Antidilutive securities excluded from computation of diluted net loss per share 2,248,953 2,248,953 242,079 64,783