Collaboration Agreements - Merck – Strategic Alliances in Infectious Diseases and Cancer Vaccines (Details)	1 Months Ended					3 Months Ended								12 Months Ended			34 Months Ended	53 Months Ended
	May 31, 2018 USD ($)	Apr. 30, 2018 USD ($)	Jun. 30, 2016 USD ($)	Jan. 31, 2016 USD ($)	Jan. 31, 2015 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2015	May 31, 2019 USD ($) candidate	Jan. 01, 2019 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from contract with customer, excluding assessed tax														$ 60,209,000
Collaboration revenue						$ 14,055,000	$ 17,046,000	$ 13,083,000	$ 16,025,000	$ 35,421,000	$ 41,757,000	$ 28,851,000	$ 29,039,000	60,209,000	$ 135,068,000	$ 205,825,000
Transaction price						249,500,000								249,500,000
Deferred revenue						199,528,000				240,924,000				199,528,000	240,924,000
Series H Redeemable Convertible Preferred Stock
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate net proceeds from the offering	$ 112,000,000
Premium recorded to deferred revenue	$ 13,000,000
Collaboration revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from contract with customer, excluding assessed tax														42,803,000
Collaboration revenue														42,803,000	76,519,000	146,953,000
Merck Agreement 2015
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Performance period																	3 years	3 years
Number of product candidates | candidate																		5
Increase (decrease) in deferred revenue																		$ 50,000,000
Upfront cash payments				$ 10,000,000
Estimated arrangement consideration																		$ 300,000,000.0
Revenue from contract with customer, excluding assessed tax															24,600,000	22,900,000
Proceeds from equity investment					$ 50,000,000
Research period																	4 years	4 years
Transaction price																			$ 65,000,000
Increase to the transaction price														(5,300,000)
Deferred revenue						0								0
Separate Agreements With Merck | Collaboration revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from contract with customer, excluding assessed tax														(4,800,000)
PCV Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Upfront cash payments			$ 200,000,000
Estimated arrangement consideration			$ 250,000,000
PCV Agreement | Series H Redeemable Convertible Preferred Stock
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Aggregate net proceeds from the offering		$ 125,000,000
Premium recorded to deferred revenue		13,000,000
PCV Agreement | PCV products
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated arrangement consideration		$ 243,000,000				243,000,000				243,000,000				243,000,000	243,000,000
PCVSAV Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																			213,000,000
Increase to the transaction price														0
Deferred revenue						83,800,000				$ 111,300,000				83,800,000	111,300,000				125,200,000
PCVSAV Agreement | Collaboration revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from contract with customer, excluding assessed tax														41,400,000	$ 41,500,000	$ 40,000,000
Milestone Payments | Merck Agreement 2015
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from contract with customer, excluding assessed tax																		$ 5,000,000
Transaction price																			5,000,000
Upfront Payment | Merck Agreement 2015
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																			60,000,000
Upfront Payment | PCVSAV Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																			200,000,000
Upfront Payment, Premium Associated With Contemporaneous Sale Of Preferred Stock | PCVSAV Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																			13,000,000
Upfront Payment, Research And Development | Merck Agreement 2015
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																			50,000,000
Upfront Payment, Funding From 2016 Amendment | Merck Agreement 2015
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																			$ 10,000,000
PCV Performance Obligation | PCVSAV Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price						206,300,000								206,300,000
KRAS Performance Obligation | PCVSAV Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price						$ 6,700,000								$ 6,700,000