Common Stock - Summary of Common Stock Shares Reserved for Future Issuance (Details) - shares	Sep. 30, 2018	Dec. 31, 2017
Class Of Stock [Line Items]
Common stock shares reserved for future issuance	86,687,472	89,292,466
Options Issued and Outstanding
Class Of Stock [Line Items]
Common stock shares reserved for future issuance	24,312,203	23,607,746
Warrant To Purchase Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Common stock shares reserved for future issuance	398,331	398,331
Warrant To Purchase Common Stock
Class Of Stock [Line Items]
Common stock shares reserved for future issuance	500,000	45,286
Conversion of Redeemable Convertible Preferred Stock
Class Of Stock [Line Items]
Common stock shares reserved for future issuance	61,279,079	61,279,079
Remaining Shares Reserved For Future Issuances Under 2014 Plan
Class Of Stock [Line Items]
Common stock shares reserved for future issuance	197,859	3,962,024