Significant agreements - Deferred revenue (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue	$ 9,863	$ 14,635	$ 14,467
Sanofi
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue	4,506	9,908	14,467
Sanofi | Hemophilia Research License and Related Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue	4,500
AstraZeneca
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue	4,549	4,727	$ 0
AstraZeneca | Target 3, Target 4, Target 5 and Target 6 Material Rights
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue	4,500
Dementia Discovery Fund
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue	$ 808	$ 0