Commitments and Contingencies - Additional Information (Details)	1 Months Ended		3 Months Ended		9 Months Ended		82 Months Ended	84 Months Ended
	Dec. 31, 2019 USD ($) ft² shares	Jan. 31, 2018 USD ($) ft²	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($) shares	Sep. 30, 2020 USD ($) shares	Dec. 31, 2013 shares
Commitments And Contingencies [Line Items]
Operating lease right-of-use assets	$ 5,775,000		$ 5,558,000		$ 5,558,000		$ 5,558,000	$ 5,558,000
Operating lease liabilities	$ 8,522,000		8,223,000		8,223,000		$ 8,223,000	$ 8,223,000
Net rent expense			$ 300,000	$ 300,000	$ 800,000	$ 800,000
Remaining lease term			8 years 9 months 18 days		8 years 9 months 18 days		8 years 9 months 18 days	8 years 9 months 18 days
Operating lease liability discount rate			10.00%		10.00%		10.00%	10.00%
Non-cancelable purchase obligations			$ 63,600,000		$ 63,600,000		$ 63,600,000	$ 63,600,000
Common stock, shares issued | shares	48,668,000		48,989,000		48,989,000		48,989,000	48,989,000
In-Licensing Agreements
Commitments And Contingencies [Line Items]
Milestone expense				$ 0	$ 3,400,000	$ 0
Accrued Royalties			$ 0		0		$ 0	$ 0
Exclusive License Agreement with The Johns Hopkins University
Commitments And Contingencies [Line Items]
Upfront and milestone payments							700,000
Common stock, shares issued | shares									88,887
Exclusive License Agreement with The Johns Hopkins University | Maximum
Commitments And Contingencies [Line Items]
Aggregate additional milestone payments							3,600,000
Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG
Commitments And Contingencies [Line Items]
Milestone payments								3,400,000
Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG | Minimum
Commitments And Contingencies [Line Items]
Annual commercial license fees								40,000
Non-exclusive License Agreement with BioWa Inc. and Lonza Sales AG | Maximum
Commitments And Contingencies [Line Items]
Aggregate additional milestone payments								38,000,000.0
2019 San Carlos Lease Agreement
Commitments And Contingencies [Line Items]
Lease agreement initiation period	2019-12
Lease term extension period	5 years
Area of office and laboratory space | ft²	98,000
Lessee operating lease contractual lease expected commencement period	2020-11
Lessee operating lease expected termination Period	2031-08
Operating lease, future minimum rental payments	$ 77,600,000
Operating lease right-of-use assets			0		0		0	0
Operating lease liabilities			0		0		0	0
Security deposit in the form of letter of credit secured by restricted cash			1,500,000		1,500,000		1,500,000	1,500,000
2018 Redwood City Lease Agreement
Commitments And Contingencies [Line Items]
Lease agreement initiation period		2018-01
Base term of lease		10 years 9 months
Lease term extension period		5 years
Area of office and laboratory space | ft²		25,000
Tenant improvement allowance		$ 1,400,000
Security deposit in the form of letter of credit secured by restricted cash			$ 800,000		$ 800,000		$ 800,000	$ 800,000