Collaboration and License Agreements - Additional Information (Details)					1 Months Ended									3 Months Ended				9 Months Ended
	May 12, 2020 shares	Apr. 29, 2020 USD ($) $ / shares shares	Apr. 01, 2020 USD ($) HumanHostFactor	Mar. 10, 2020 USD ($)	Jun. 30, 2020 USD ($)	Apr. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Aug. 31, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jul. 31, 2018 USD ($)	May 31, 2018 USD ($)	Oct. 31, 2017 USD ($) shares	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of common stock, value														$ 323,214,000				$ 323,214,000
License
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue																		22,747,000
Stock Purchase Agreement | GGL
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of common stock, shares | shares		6,626,027
Share purchase price per share | $ / shares		$ 37.73
Issuance of common stock, value		$ 250,000,000.0
Preliminary Collaboration Agreement | GSK
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of years conduct certain research and development activities under mutually agreed development plans		4 years
Maximum percentage of right to perform details in connection with antibody product		20.00%
Addition (reduction) of research and development expense														1,200,000				(2,600,000)
Preliminary Collaboration Agreement | GSK | Accrued Liabilities
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payable														2,600,000				$ 2,600,000
Preliminary Collaboration Agreement | Antibody Program | GSK
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of development costs		72.50%
Preliminary Collaboration Agreement | Vaccine Program | GSK
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of development costs		27.50%
Preliminary Collaboration Agreement and Stock Purchase Agreement | GSK
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Premium paid on sale price of common stock		$ 43,300,000
Total revenue		43,300,000
Preliminary Collaboration Agreement and Stock Purchase Agreement | GGL | ASC 606
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Fair market value of the common stock issued		$ 206,700,000
Common stock price per share | $ / shares		$ 36.70
Premium paid on sale price of common stock		$ 43,300,000
Brii Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Written notice period for not exercising an option												30 days
Written notice period for exercise an option												180 days
Written notice period for uncured material breach												60 days
Written notice period for failure to make payment												30 days
Brii Agreement | Brii Bio Parent
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of ordinary share equal to outstanding share												9.90%
Option exercise fee ranging from mid-single-digit					$ 20,000,000.0										$ 20,000,000.0
Sales milestone receivable					175,000,000.0										175,000,000.0
Estimated fair value of investments												$ 6,600,000
Contract liability					2,700,000										2,700,000
Option exercise fee received					20,000,000.0
Determined transaction price					22,700,000
Brii Agreement | Brii Bio
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Royalty payment obligation expiration period after first commercial sales																		10 years
Contract liability												6,600,000
Brii Agreement | Brii Bio | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Option exercise fee ranging from mid-single-digit												20,000,000.0
Option exercise fee of ranging from low tens												50,000,000.0
Brii Agreement | Brii Bio | China Territory | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sales milestone receivable												175,000,000.0
Brii Agreement | Brii Bio | United States | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Sale milestone payments												175,000,000.0
Brii Agreement | Brii Bio | Other Assets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Carrying value of investment														5,700,000				$ 5,700,000		$ 6,600,000
Brii Agreement | Alnylam Pharmaceuticals Inc | Research and Development Expenses
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Proceeds from options exercised															10,000,000.0
Brii Agreement | Licensed Product-By-Licensed Product | Brii Bio Parent
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment on basis ranging from mid-single-digit					30,000,000.0										30,000,000.0
Brii Agreement | Licensed Product-By-Licensed Product | Brii Bio | Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment on basis ranging from mid-single-digit												30,000,000.0
Milestone payment on basis of ranging from low tens												$ 100,000,000.0
Brii Agreement | License | Brii Bio Parent
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total revenue															22,700,000
Alnylam Agreement | Alnylam Pharmaceuticals Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Issuance of common stock, shares | shares	1,111,111												1,111,111
Royalty payment obligation expiration period after first commercial sales																		10 years
Written notice period for exercise an option													90 days
Written notice period for uncured material breach													60 days
Written notice period for failure to make payment													30 days
Upfront fee paid													$ 10,000,000.0
Recognized liability																				800,000
Maximum shares to be issued | shares													1,111,111
Written notice period for licensed program other party challenges validity or enforceability of patent license.													30 days
Embedded Derivative liability								$ 0									$ 0		$ 0	13,600,000
Estimated fair value of the derivative liability				$ 29,200,000																$ 12,400,000
Obligation payment							$ 15,000,000.0									$ 15,000,000.0
Milestone shares period issued				60 days
Maximum aggregate milestone payments						$ 15,000,000.0
Maximum milestone payment for achievement of specified development							$ 15,000,000.0
Maximum number of human host factor targets related to susceptibility to corona viruses | HumanHostFactor			3
Maximum milestone payment payable for achievement of specified development			$ 15,000,000.0
Milestone payments paid.															$ 10,000,000.0	$ 15,000,000.0
Expenses incurred under agreement														800,000			7,500,000	$ 5,600,000	10,700,000
Alnylam Agreement | First siRNA product | Alnylam Pharmaceuticals Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum milestone payment for achievement of specified development and regulatory													$ 190,000,000.0
Alnylam Agreement | Each Infectious Disease siRNA | Alnylam Pharmaceuticals Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum milestone payment for achievement of specified development and regulatory													115,000,000.0
Alnylam Agreement | Commercialization | Alnylam Pharmaceuticals Inc
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum milestone payment for achievement of specified development and regulatory													100,000,000.0
Maximum aggregate milestone payment for achievement of specified development and regulatory													$ 250,000,000.0
Development and Manufacturing Collaboration Agreement | WuXi Biologics
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Termination description																		the WuXi Biologics Collaboration Agreement may be terminated by (i) the written agreement of both parties, (ii) WuXi Biologics following the one year anniversary of the WuXi Biologics Collaboration Agreement effective date with respect to the entire agreement or on a product by product basis with 90 days’ prior written notice or (iii) by either party if the other party materially breaches the WuXi Biologics Collaboration Agreement and fails to cure such breach within sixty days.
Rockefeller Agreement | Rockefeller
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront fee paid											$ 300,000
Maximum aggregate milestone payment for achievement of specified development and regulatory											40,000,000.0
Royalty Obligation Period From Date Of First Commercial Sale																		12 years
Rockefeller Agreement | Commercialization | Rockefeller
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Payment of annual license maintenance fees											1,000,000.0
Rockefeller Agreement | First Infectious Disease Product | Rockefeller
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum milestone payment for achievement of specified development and regulatory											8,500,000
Rockefeller Agreement | First Four Other Infectious Disease Products | Rockefeller
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum milestone payment for achievement of specified development and regulatory											7,000,000.0
Rockefeller Agreement | Any Other Infectious Disease Product | Rockefeller
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum milestone payment for achievement of specified development and regulatory											$ 3,600,000
Rockefeller Agreement | HBV Program | Rockefeller | Research and Development Expenses
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Specified development milestones and annual license maintenance fees																		$ 1,300,000
License maintenance fees														$ 0			$ 0		$ 1,000,000.0
2018 MedImmune Agreement | MedImmune
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
License agreement upfront payment										$ 10,000,000.0
2018 MedImmune Agreement | Influenza A and Influenza B | MedImmune
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Development, regulatory, and commercial milestone payments maximum amount										$ 331,500,000
2018 MedImmune Agreement | Influenza A | MedImmune
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Specified development milestones payment								5,000,000.0
2019 Xencor Agreement | Xencor
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Written notice period for uncured material breach																		60 days
Written notice period for failure to make payment																		30 days
Written notice period for termination of licensed program																		60 days
Written notice period for termination of licensed program based on challenge																		30 days
2019 Xencor Agreement | Influenza A Research Programs | Xencor
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum aggregate milestone payments									$ 77,800,000
Specified development milestones payment								$ 800,000
Maximum aggregate development and regulatory milestone payments									17,800,000
Maximum aggregate commercial sales milestone payments									60,000,000.0
2019 Xencor Agreement | HBV Research Programs | Xencor
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum aggregate milestone payments									77,800,000
Specified development milestones payment					$ 300,000
2019 Xencor Agreement | Influenza A and HBV Research Programs | Xencor
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Maximum aggregate milestone payments									$ 155,500,000
2020 Xencor Agreement | Xencor
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Written notice period for uncured material breach																		60 days
Written notice period for failure to make payment																		30 days
Licensed patents expiration period																		12 years
Written notice period for termination of licensed program																		60 days
Written notice period for termination of licensed program based on challenge																		30 days