Significant agreements - Deferred revenue (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020	Dec. 31, 2019
Deferred revenue
Deferred revenue	$ 62,507	$ 35,156	$ 5,657
Deferred revenue material rights option
Deferred revenue
Deferred revenue	3,900
Ionis
Deferred revenue
Deferred revenue	34,560	3,000
Genentech
Deferred revenue
Deferred revenue	25,090	27,579	0
Genentech | Deferred revenue material rights option
Deferred revenue
Deferred revenue	21,800
AstraZeneca
Deferred revenue
Deferred revenue	2,354	$ 3,756	$ 4,913
AstraZeneca | Target 4 and Target 5 Material Rights
Deferred revenue
Deferred revenue	$ 2,400