Collaboration Agreements - AstraZeneca - Strategic Alliances in Cardiovascular and Oncology (Details)	1 Months Ended				3 Months Ended								12 Months Ended
	Jun. 30, 2018 option	Oct. 31, 2017	Jan. 31, 2016	Mar. 31, 2013 USD ($) milestone option	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($) option	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Jan. 01, 2019 USD ($)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price					$ 249,500,000								$ 249,500,000
Deferred revenue					199,528,000				$ 240,924,000				199,528,000	$ 240,924,000
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
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
Combined 2018 AZ Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																	$ 400,000,000
Increase to the transaction price						1,000,000							1,800,000
Change in estimate of variable consideration													800,000
Deferred revenue					73,700,000				115,600,000				73,700,000	115,600,000			75,700,000
Combined 2018 AZ Agreements | Collaboration revenue
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Revenue from contract with customer, excluding assessed tax													4,300,000	45,400,000	$ 30,000,000
2013 AZ Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																	240,000,000
Number of options to develop candidates | option				40
Upfront cash payments				$ 240,000,000												$ 10,000,000
Estimated arrangement consideration				$ 180,000,000.0
Number of milestones | milestone				3
Option exercise fee				$ 10,000,000
Termination period				90 days
2013 AZ Agreements | VEGF-A product (AZD8601)
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Upfront cash payments														30,000,000
2013 AZ Agreements | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Earn-out payments				12.00%
2013 AZ Agreements | Development Milestones
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated arrangement consideration				$ 100,000,000.0
2013 AZ Agreements | Regulatory Milestones
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated arrangement consideration				100,000,000.0
2013 AZ Agreements | Commercial Milestones
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Estimated arrangement consideration				$ 200,000,000.0
VEGF Exercise 2016
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																	55,100,000
Increase to the transaction price						$ 2,300,000							2,800,000
Change in estimate of variable consideration													500,000
Deferred revenue					41,200,000				$ 41,200,000				41,200,000	41,200,000			37,100,000
2016 AZ Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Termination period			90 days
Suspension period			12 months
2016 AZ Agreement | Minimum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Tiered royalties			10.00%
2016 AZ Agreement | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Tiered royalties at default rate			20.00%
Tiered royalties			30.00%
2017 AZ Agreement
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Termination period		90 days
Suspension period		12 months
Collaboration revenue													0	$ 600,000
2017 AZ Agreement | Minimum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Tiered royalties		10.00%
2017 AZ Agreement | Maximum
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Tiered royalties		30.00%
2018 A&R Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Number of options to develop candidates | option	40										40
Termination period	90 days
Variable Consideration | Combined 2018 AZ Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																	160,000,000
Variable Consideration | VEGF Exercise 2016
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																	15,100,000
Estimated Reimbursement | Combined 2018 AZ Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																	40,000,000
Estimated Reimbursement | Combined 2018 AZ Agreements | Toxicity Milestone
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price					60,000,000								60,000,000
Fixed Payments | VEGF Exercise 2016
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																	40,000,000
Milestone Payments | Combined 2018 AZ Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																	120,000,000
Milestone Payments | Combined 2018 AZ Agreements | Toxicity Milestone
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																	60,000,000
Milestone Payments | Combined 2018 AZ Agreements | Competition Milestone
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																	60,000,000
Milestone Payments | VEGF Exercise 2016
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																	30,000,000
Option Exercise Fee | VEGF Exercise 2016
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price																	$ 10,000,000
Combined 2018 AZ Agreement Performance Obligation | Combined 2018 AZ Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price					293,200,000								293,200,000
IL12 Performance Obligation | Combined 2018 AZ Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price					8,100,000								8,100,000
Oncology Development Target Performance Obligation | Combined 2018 AZ Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price					8,100,000								8,100,000
Development And Commercialization License And Manufacturing Obligations For IL12 | Combined 2018 AZ Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price					90,700,000								90,700,000
Development And Commercialization Rights And Manufacturing Services For An Oncology Development Target | Combined 2018 AZ Agreements
Research and Development Arrangement, Contract to Perform for Others [Line Items]
Transaction price					$ 1,600,000								$ 1,600,000