Significant agreements - Sanofi Collaboration Agreement (Details) - USD ($) $ in Thousands	1 Months Ended	3 Months Ended
	Aug. 31, 2017	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018
Significant agreements.
Revenue recognized by non exercising of option		$ 0	$ 5,340
Revenue from collaborative arrangement		1,129	6,384
Deferred revenue		34,253		$ 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	5,959
Deferred revenue		$ 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