Significant agreements - Sanofi Collaboration Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	Aug. 31, 2017	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Significant agreements.
Revenue recognized by non exercising of option										$ 1,702	$ 9,984
Revenue from collaborative arrangement		$ 3,848	$ 3,842	$ 1,571	$ 1,129	$ 5,281	$ 614	$ 1,522	$ 6,384	10,390	13,801	$ 7,136
Deferred revenue		35,156				5,657				35,156	5,657	14,635
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										0	10,724	4,007
Deferred revenue		$ 0				0				$ 0	0	$ 9,908
Sanofi | Sickle Cell Research License and Related Services
Significant agreements.
Revenue recognized by non exercising of option											5,300
Sanofi | Hemophilia Research License and Related Services
Significant agreements.
Revenue recognized by non exercising of option											$ 4,700
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