Common Stock - Shares of Common Stock Reserved for Future Issuance (Details) - shares	Sep. 30, 2020	Dec. 31, 2019
Class Of Stock [Line Items]
Shares of common stock reserved for future issuance	14,280,326	13,781,623
Series A Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Shares of common stock reserved for future issuance	4,247,153	4,247,153
Series B Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Shares of common stock reserved for future issuance	2,633,138	2,625,142
Series C Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Shares of common stock reserved for future issuance	2,584,633	2,584,633
Common Stock Reserved for Exercises of Outstanding Stock Options Issued under 2015 Stock Option and Grant Plan, as Amended
Class Of Stock [Line Items]
Shares of common stock reserved for future issuance	4,541,345	4,251,914
Common Stock Reserved for Issuances under 2015 Stock Option and Grant Plan, as Amended
Class Of Stock [Line Items]
Shares of common stock reserved for future issuance	274,057	72,781