Significant agreements - Sanofi Collaboration Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Aug. 31, 2017	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Significant agreements.
Revenue recognized by non exercising of option		$ 1,876	$ 1,702	$ 9,984
Revenue from collaborative arrangement		11,697	10,390	13,801
Deferred revenue		$ 71,340	$ 35,156	5,657
Sanofi
Significant agreements.
Upfront cash payment	$ 10,000
Non refundable payment for services during the BV Bicycle Research Term	$ 4,200
Revenue from collaborative arrangement				10,724
Sanofi | Sickle Cell License Option Material Right
Significant agreements.
Revenue recognized by non exercising of option				5,300
Sanofi | Hemophilia License Option Material Right
Significant agreements.
Revenue recognized by non exercising of option				$ 4,700