Commitments and Contingencies - Operating Lease (Details) ft² in Thousands, $ in Thousands	3 Months Ended
	Mar. 31, 2020 USD ($) ft²	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)
Future minimum lease payments under non-cancelable operating leases
Remainder of 2020	$ 1,760
2020	2,401
2021	2,472
2022	2,543
2023	2,614
Thereafter	4,227
Total future minimum lease payments	16,017
Less: amount representing interest	3,435
Present value of future lease payments	12,582
Less: short-term lease liabilities	1,450		$ 1,407
Long-term lease liabilities	11,132		11,506
Operating lease expense	$ 474	$ 476
Headquarters and laboratory facilities, South San Francisco, lease signed April 2015
Operating Leases
Amount of space leased (in square feet) | ft²	59
Security deposit	$ 603		603
Other assets | Laboratory facilities, Austin, Texas
Operating Leases
Security deposit	$ 139		$ 139