Significant agreements - Genetech Collaboration Agreement (Details) $ in Thousands			3 Months Ended								12 Months Ended
	Feb. 21, 2021 USD ($) item	Feb. 21, 2020 USD ($) item	Dec. 31, 2020 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Mar. 31, 2021 USD ($)
Significant agreements.
Collaboration revenues			$ 3,848	$ 3,842	$ 1,571	$ 1,129	$ 5,281	$ 614	$ 1,522	$ 6,384	$ 10,390	$ 13,801	$ 7,136
Deferred revenue			35,156				$ 5,657				$ 35,156	$ 5,657	$ 14,635
Genentech
Significant agreements.
Number of potential development candidates | item	4
Upfront cash payment	$ 30,000	$ 30,000
Number of immuno oncology targets | item	2
Additional number of immuno oncology targets | item	2
Expansion Fee	$ 10,000
Transaction price		31,000
Period over which performance obligations will be performed											2 years
Material rights exercise period											4 years
Collaboration revenues											$ 4,896
Deferred revenue			$ 27,579								$ 27,579
Genentech | Subsequent events
Significant agreements.
Sales milestone payments, receivable														$ 2,000
Genentech | Royalty
Significant agreements.
Number of years over which royalty is payable											10 years
Genentech | Maximum
Significant agreements.
Regulatory, and initial commercialization milestones, 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		3,775
Genentech | Collaboration Program 2 Performance Obligation
Significant agreements.
Transaction price		$ 7,550
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		$ 330
Genentech | Two material rights associated with the LSR Go Option for Collaboration Programs One And Two
Significant agreements.
Number of collaboration programs | item		2
Transaction price		$ 11,650
Genentech | Material rights for associated and limited substitution rights
Significant agreements.
Transaction price		$ 1,115
Genentech | Two material rights for Expansion Options
Significant agreements.
Number of expansion option collaboration programs | item		2
Transaction price		$ 6,580
Genentech | Collaboration Program One and Two Performance Obligation | Maximum
Significant agreements.
Sales milestone payments, receivable		$ 200,000