Preferred Stock Warrants - Summary of Outstanding Redeemable Convertible Preferred Stock Warrants and Corresponding Exercise Price (Details) - Redeemable Convertible Preferred Stock - $ / shares	Sep. 30, 2018	Aug. 31, 2018	Dec. 31, 2017
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding	708,484		486,356
Series AA Warrants
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding			1,506
Exercise Price	$ 13.28
Expiration Date	Mar. 07, 2018
2009 Series B Warrants
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding	25,884		25,884
Exercise Price	$ 9.659
Expiration Date	Dec. 22, 2019
2014 Series B Warrants
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding	104,856		104,856
Exercise Price	$ 0.12
Expiration Date	May 14, 2021
Convertible Debt Series C-1 Warrants
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding	304,397		304,397
Exercise Price	$ 0.12
Expiration Date	Jan. 17, 2024
Term Loan Series C-1 Warrants
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding	49,713		49,713
Exercise Price	$ 9.659
Expiration Date	Oct. 18, 2027
2018 Series C-1 Warrants
Class Of Warrant Or Right [Line Items]
Number of Shares Outstanding	223,634
Exercise Price	$ 0.12	$ 0.12
Expiration Date	Aug. 25, 2025