Significant agreements - Deferred revenue (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018	Dec. 31, 2017
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue	$ 10,370	$ 14,635	$ 14,467
Sanofi
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue	4,656	9,908	14,467
Sanofi | Hemophilia Research License and Related Services
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue	4,700
AstraZeneca
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue	4,702	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	4,700
Dementia Discovery Fund
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue	$ 1,012	$ 0