Significant agreements - Evaluation and Option Agreement (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Collaboration revenues	$ 1,785	$ 1,571	$ 3,593	$ 2,700
Deferred revenue	34,074		$ 34,074		$ 35,156	$ 5,657
Ionis Evaluation and Option Agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Term, Bicycle Research Team (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
Collaboration revenues			0		3,000
Deferred revenue	3,039		3,039		$ 3,000
Transaction price	$ 3,000		$ 3,000