Significant agreements - Sanofi Collaboration Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended
	Aug. 31, 2017	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Dec. 31, 2018
Significant agreements.
Revenue recognized by non exercising of option		$ 0	$ 0		$ 5,286
Revenue from collaborative arrangement		3,842	614	$ 6,542	8,520
Deferred revenue		33,275		33,275		$ 5,657	$ 14,635
Development milestone
Significant agreements.
Non refundable payment for services during the BV Bicycle Research Term				4,200
Sanofi
Significant agreements.
Total transaction price initially determined	$ 14,200					14,900
Upfront cash payment	10,000
Non refundable payment for services during the BV Bicycle Research Term	4,200
Revenue from collaborative arrangement		0	0	0	$ 6,016
Deferred revenue		$ 0		$ 0		$ 0	$ 9,908
Sanofi | Sickle Cell Research License and Related Services
Significant agreements.
Total transaction price initially determined	1,405
Sanofi | Hemophilia Research License and Related Services
Significant agreements.
Total transaction price initially determined	2,811
Sanofi | Sickle Cell License Option Material Right
Significant agreements.
Total transaction price initially determined	5,286
Revenue recognized by non exercising of option			$ 5,300
Sanofi | Hemophilia License Option Material Right
Significant agreements.
Total transaction price initially determined	$ 4,698