Significant agreements - Ionis (Details) $ in Thousands		3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 09, 2021 USD ($) item	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($) item	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration revenues		$ 4,333	$ 3,842	$ 7,926	$ 6,542
Deferred revenue		62,507		62,507		$ 35,156	$ 5,657
Ionis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total transaction price initially determined	$ 38,000	38,000		38,000
Upfront payment	$ 31,000
Estimated amount payable to CROs		600		$ 600
Number of collaboration programs | item				4
Number of collaboration targets | item	4
Expected period for satisfaction of performance obligations				3 years
Material rights exercise period				4 years
Collaboration revenues		2,066	$ 0	$ 2,066	$ 0
Deferred revenue		34,560		34,560		3,000
Ionis | Combined licenses and research and discovery performance obligation
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total transaction price initially determined		34,100		34,100
Ionis | Four material rights associated with credits for IND Acceptance fees
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total transaction price initially determined		$ 3,900		3,900
Ionis Evaluation and Option Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront cash payment				$ 3,000		3,000
Collaboration revenues						3,000
Deferred revenue						$ 3,000	$ 0