Significant agreements - Genentech Collaboration Agreement (Details) $ in Thousands		3 Months Ended		6 Months Ended
	Feb. 21, 2020 USD ($) item	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Jun. 30, 2021 USD ($) item	Jun. 30, 2020 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Significant agreements.
Collaboration revenues		$ 1,785	$ 1,571	$ 3,593	$ 2,700
Deferred revenue		34,074		34,074			$ 35,156	$ 5,657
Genentech
Significant agreements.
Number of potential development candidates | item	4
Upfront cash payment	$ 30,000
Number of immuno oncology targets | item	2
Additional number of immuno oncology targets | item	2
Expansion Fee	$ 10,000
Sales milestone payments, receivable						$ 2,000
Transaction price	31,000	33,000		$ 33,000		$ 33,000
Period over which performance obligations will be performed				2 years
Material rights exercise period				4 years
Collaboration revenues		1,631	$ 1,196	$ 3,079	$ 1,857
Deferred revenue		26,867		$ 26,867			$ 27,579
Genentech | Royalty
Significant agreements.
Number of years over which royalty is payable				10 years
Genentech | Maximum
Significant agreements.
Sales milestone payments, receivable	200,000
Genentech | Development milestone | Maximum
Significant agreements.
Sales milestone payments, receivable	65,000
Genentech | Regulatory milestone | Maximum
Significant agreements.
Sales milestone payments, receivable	$ 135,000
Genentech | Collaboration Program 1 Performance Obligation
Significant agreements.
Transaction price		4,019		$ 4,019
Genentech | Collaboration Program 2 Performance Obligation
Significant agreements.
Transaction price		8,037		8,037
Genentech | 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		352		$ 352
Genentech | Two material rights associated with the LSR Go Option for Collaboration Programs One And Two
Significant agreements.
Number of expansion option collaboration programs | item				2
Number of collaboration programs | item	2			2
Transaction price		12,400		$ 12,400
Genentech | Material rights for associated and limited substitution rights
Significant agreements.
Transaction price		1,187		1,187
Genentech | Two material rights for Expansion Options
Significant agreements.
Number of expansion option collaboration programs | item	2
Transaction price		$ 7,005		$ 7,005
Genentech | Collaboration Program One and Two Performance Obligation | Maximum
Significant agreements.
Regulatory, and initial commercialization milestones, payments receivable	$ 200,000