Operating Lease - Additional Information (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019 USD ($) ft²	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) ft²	Sep. 30, 2018 USD ($)
Operating lease existence of option to extend [true false]			true
Operating lease, option to extend			The operating lease agreement has one option to extend the lease term for a period of five years at the fair market rate at the time of the extension.
Operating lease, renewal term	5 years		5 years
Weighted average remaining lease term	5 years 5 months 1 day		5 years 5 months 1 day
Operating lease liability, weighted average discount rate	10.00%		10.00%
Rent expense		$ 300		$ 700
Variable lease costs	$ 148	$ 100	$ 392	$ 300
South San Francisco, California [Member]
Area of lease | ft²	24,357		24,357
Lease Expiration Date			Feb. 28, 2025