Research and Collaboration Agreements - Bristol-Myers Squibb Company - Additional Information (Details)				3 Months Ended			6 Months Ended
	Apr. 25, 2017 USD ($) Target SaleIndicator	Jul. 07, 2014 USD ($) Target Term ResearchTarget SaleIndicator	May 23, 2014 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Nov. 30, 2017 USD ($)	Mar. 17, 2017 USD ($) Target	Dec. 31, 2016 USD ($)	Jan. 31, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement				$ 9,013,000		$ 21,338,000	$ 38,498,000	$ 35,522,000
Type of Revenue [Extensible List]				us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue				$ 249,510,000			$ 249,510,000		$ 277,980,000
Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue				172,800,000			$ 172,800,000
Collaborative Arrangement | Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration target research term		2 years
Number of additional collaboration target | Target		2
Research terms							Each collaboration target had a two-year research term and the two additional targets had to be nominated by BMS within five years of the effective date of the BMS Agreement. The research term for each collaboration target could be extended in one year increments up to three times.
Extension of research term for each collaboration target		1 year
Upfront payment received	$ 200,000,000.0	$ 50,000,000.0	$ 250,000,000.0
Contingent milestone payments receivable												$ 15,000,000.0	$ 10,000,000.0
Revenue recognition upon performance obligation			272,800,000
Upfront payment received			10,800,000
Milestone payment received			$ 12,000,000.0
Estimated research service termination date							Apr. 25, 2025
Revenue recognized from collaborative arrangement				$ 7,400,000		$ 8,200,000	$ 32,900,000	$ 16,400,000
Type of Revenue [Extensible List]				us-gaap:LicenseAndServiceMember		us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue				$ 172,800,000			$ 172,800,000		205,600,000
Amount due from agreement				$ 4,000,000,000			4,000,000,000		$ 100,000
Collaborative Arrangement | Bristol Myers Squibb Company | Clinical Candidate
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable												$ 2,000,000.0
Collaborative Arrangement | Bristol Myers Squibb Company | New Drug Application for CTLA-4
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable										$ 10,000,000.0
Collaborative Arrangement | Bristol Myers Squibb Company | Third And Fourth Immuno-Oncology Targets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Revenue recognized from collaborative arrangement					$ 17,400,000		$ 17,400,000
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of oncology target | Target		4
Number of collaboration target | Target		2
Period of nomination of additional target from effective date		5 years
Times of increments for extended collaboration target research time | Term		3
Aggregate future contingent milestone payment		$ 1,217,000,000.0									$ 3,586,000,000.0
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Each Of Two Additional Targets
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment		25,000,000.0
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achievement Of Development Milestones For Each Research Target Program
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	116,000,000.0	114,000,000.0
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achievement Of Development Milestones If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment		$ 456,000,000.0
Number of research targets selected | ResearchTarget		4
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving First Commercial Sale In Various Territories
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 124,000,000.0	$ 124,000,000.0
Number of sales indicators | SaleIndicator	3	3
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving First Commercial Sale If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment		$ 496,000,000.0
Number of research targets selected | ResearchTarget		4
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestones For Each Research Target Program
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 60,000,000.0	$ 60,000,000.0
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestones If Four Research Targets Selected By Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment		$ 240,000,000.0
Number of research targets selected | ResearchTarget		4
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achievement of Development Milestones if Eight Research Targets Selected by Counterparty
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 928,000,000.0
Number of research targets selected | Target	8
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Eight Targets Selected for the First Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 992,000,000.0
Number of research targets selected | Target	8
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestones if Eight Research Targets Selected for First Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 480,000,000.0
Number of research targets selected | Target	8
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Development Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 56,300,000
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Development Milestone Payment if Eight Targets are Selected for Second Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 450,000,000.0
Number of research targets selected | Target	8
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Milestone Payments for First Commercial Sale in Various Territories for Up to Three Indication
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 62,000,000.0
Number of sales indicators | SaleIndicator	3
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Development Milestone Payment if Eight Targets are Selected for Second Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 496,000,000.0
Number of research targets selected | Target	8
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 30,000,000.0
Collaborative Arrangement | Maximum | Bristol Myers Squibb Company | Achieving Sales Milestone Payment if Eight Research Target are selected for Second Product Modality
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Aggregate future contingent milestone payment	$ 240,000,000.0
Number of research targets selected | Target	8
Collaboration and License Agreement | Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of non-oncology target | Target											2
Collaboration and License Agreement | Maximum | Bristol Myers Squibb Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of oncology target | Target											6