Collaboration Agreements - Merck – Strategic Alliances in Infectious Diseases and Cancer Vaccines (Details)	1 Months Ended					3 Months Ended		6 Months Ended		34 Months Ended	53 Months Ended
	Jan. 31, 2019	Apr. 30, 2018 USD ($)	Jun. 30, 2016 USD ($)	Jan. 31, 2016 USD ($)	Jan. 31, 2015 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2015	May 31, 2019 USD ($) candidate	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Upfront payment								$ (23,100,000)	$ (2,283,000)
Total collaboration revenue (contra-revenue)						$ 13,083,000		29,108,000
Deferred revenue						218,051,000		218,051,000					$ 240,924,000
Amortization of deferred revenue due to the satisfaction of our performance obligation during the period						11,583,000		25,165,000
Collaboration revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Total collaboration revenue (contra-revenue)						9,842,000		23,143,000
2015 Merck Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Performance period	3 years									3 years	3 years
Research period										4 years	4 years
Number of product candidates | candidate											5
Upfront payment											$ 50,000,000
Estimated arrangement consideration											300,000,000.0
Total collaboration revenue (contra-revenue)							$ 6,800,000		12,700,000
Proceeds from equity investment					$ 50,000,000
Upfront cash payments				$ 10,000,000
Decrease to the transaction price								2,100,000
Deferred revenue						0		0					0
2015 Merck Agreement | Milestone Payments
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Total collaboration revenue (contra-revenue)											$ 5,000,000
Separate Agreements With Merck | Collaboration revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Total collaboration revenue (contra-revenue)						(1,800,000)		(1,300,000)
PCV Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated arrangement consideration			$ 250,000,000
Upfront cash payments			$ 200,000,000
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
PCV/SAV Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Decrease to the transaction price								0
Deferred revenue						104,500,000		104,500,000				$ 125,200,000	$ 111,300,000
PCV/SAV Agreement | PCV products
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated arrangement consideration		$ 243,000,000
PCV/SAV Agreement | Collaboration revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Total collaboration revenue (contra-revenue)						$ 10,400,000	$ 10,300,000	$ 20,600,000	$ 20,300,000