Significant agreements - Genentech Collaboration Agreement (Details) $ in Thousands			3 Months Ended
	Feb. 21, 2021 USD ($) item	Feb. 21, 2020 USD ($) item	Mar. 31, 2021 USD ($) item	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Significant agreements.
Collaboration revenues			$ 1,808	$ 1,129
Deferred revenue			35,710		$ 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
Period over which performance obligations will be performed			2 years
Material rights exercise period			4 years
Collaboration revenues			$ 1,448	$ 661
Deferred revenue			$ 28,377		$ 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
Genentech | Collaboration Program 2 Performance Obligation
Significant agreements.
Transaction price			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
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
Genentech | Material rights for associated and limited substitution rights
Significant agreements.
Transaction price			1,187
Genentech | Two material rights for Expansion Options
Significant agreements.
Number of expansion option collaboration programs | item		2
Transaction price			$ 7,005
Genentech | Collaboration Program One and Two Performance Obligation | Maximum
Significant agreements.
Regulatory, and initial commercialization milestones, payments receivable		$ 200,000