Significant agreements - Ionis Collaboration Agreement (Details) $ in Millions		9 Months Ended	12 Months Ended
	Jul. 09, 2021 USD ($) item	Sep. 30, 2021 USD ($)	Dec. 31, 2020 USD ($)
Ionis
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Period for which research and discovery activities will be performed with no additional consideration	3 years
Upfront payment | $	$ 31.0
Number of payments of a mid-single-digit million dollar, failing to achieve the specified development diligence | item	3
Number of collaboration targets | item	4
Number of years over which royalty is payable	10 years
Ionis Evaluation and Option Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront cash payment | $		$ 3.0	$ 3.0