Significant agreements - Ionis Evaluation and Option Agreement (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
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 Evaluation and Option Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Term, Bicycle Research Term (in months)					4 months
Threshold period for receipt of revenue after receipt of the materials and related data package					5 days
Threshold period of notice required for termination of agreement			30 days
Upfront cash payment			$ 3,000		$ 3,000
Collaboration revenues					3,000
Deferred revenue					$ 3,000	$ 0