Significant agreements - Genetech Collaboration Agreement (Details) $ in Thousands		3 Months Ended
	Feb. 21, 2020 USD ($) item	Mar. 31, 2020 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Significant agreements.
Collaboration revenues		$ 1,129	$ 6,384
Deferred revenue		34,253		$ 5,657	$ 14,635
Genetech
Significant agreements.
Number of Potential development candidates | item	4
Upfront cash payment	$ 30,000
Number of immuno oncology targets | item	2
Expansion Fee	$ 10,000
Number of collaboration programs | item	2
Transaction price	$ 31,000
Collaboration revenues		661	$ 0
Deferred revenue		$ 28,955
Period over which performance obligations will be performed		2 years
Material rights exercise period		4 years
Genetech | Royalty
Significant agreements.
Number of years over which royalty is payable		10 years
Genetech | Maximum
Significant agreements.
Additional number of immuno oncology targets | item	2
Regulatory, and initial commercialization milestones, payments receivable	$ 200,000
Genetech | Development milestone | Maximum
Significant agreements.
Sales milestone payments, receivable	65,000
Genetech | Regulatory milestone | Maximum
Significant agreements.
Sales milestone payments, receivable	135,000
Genetech | Collaboration Program 1 Performance Obligation
Significant agreements.
Transaction price	3,775
Genetech | Collaboration Program 2 Performance Obligation
Significant agreements.
Regulatory, and initial commercialization milestones, payments receivable	8,000,000,000
Transaction price	$ 7,550
Genetech | Specified Targeting Arm Material Right Arm Program One
Significant agreements.
Number of initial collaboration programs exercised | item	1
Sales milestone payments, receivable	$ 1,000
Transaction price	330
Genetech | Two material rights associated with the LSR Go Option for Collaboration Programs One And Two
Significant agreements.
Transaction price	11,650
Genetech | Material rights for associated and limited substitution rights
Significant agreements.
Transaction price	$ 1,115
Genetech | Two material rights for Expansion Options or Dev Go Option
Significant agreements.
Number of expansion option collaboration programs | item	2
Transaction price	$ 6,580
Genetech | Collaboration Program One and Two Performance Obligation | Maximum
Significant agreements.
Sales milestone payments, receivable	$ 200,000
Genetech | Material rights
Significant agreements.
Deferred revenue		$ 19,700