Significant agreements - Sanofi Collaboration Agreement (Details) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Mar. 31, 2018	Aug. 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue recognized by non exercising of option			$ 0	$ 0	$ 5,286	$ 0
Revenue from collaborative arrangement			614	1,610	8,520	6,079
Deferred revenue			9,863		9,863		$ 14,635	$ 14,467
Development milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non refundable payment for services during the BV Bicycle Research Term					4,200
Sanofi
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Term of research program		3 years
Renewal period of Research		1 year
Upfront cash payment		$ 10,000
Non refundable payment for services during the BV Bicycle Research Term		$ 4,200
Threshold period of notice required for termination of agreement		30 days
Total transaction price initially determined		$ 14,200	14,900		14,900
Transaction price		14,200	14,900		14,900
Revenue from collaborative arrangement			0	$ 1,047	6,016	$ 3,258
Deferred revenue			4,506		4,506		$ 9,908	$ 14,467
Sanofi | Sickle Cell Research License and Related Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non refundable payment for services during the BV Bicycle Research Term	$ 1,400
Total transaction price initially determined		1,405
Transaction price		1,405
Sanofi | Hemophilia Research License and Related Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non refundable payment for services during the BV Bicycle Research Term	$ 2,800
Total transaction price initially determined		2,811
Transaction price		2,811
Deferred revenue			$ 4,500		4,500
Sanofi | Sickle Cell License Option Material Right
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total transaction price initially determined		5,286
Transaction price		5,286
Revenue recognized by non exercising of option					$ 5,300
Sanofi | Hemophilia License Option Material Right
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Total transaction price initially determined		4,698
Transaction price		4,698
Sanofi | Development milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Option fee payable in order to obtain worldwide development and exploitation rights		5,000
Sanofi | Development milestone | Sickle Cell License Option Material Right
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone payments		47,500
Sanofi | Development milestone | Hemophilia License Option Material Right
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone payments		67,000
Sanofi | Regulatory milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone payments		104,000
Sanofi | Commercial milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone payments		$ 55,000