Summary of significant accounting policies - Leases and Revenue recognition from contracts with customers (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) item Program	Jan. 01, 2019 USD ($)
Summary of significant accounting policies
Number of preclinical research programs | Program	2
Number of combined performance obligation for each research program | item	1
Number of research program performance obligations | item	2
Operating lease assets	$ 7,875
Operating lease liability	$ 7,104
Adjustment | ASU 2016-02
Summary of significant accounting policies
Operating lease assets		$ 3,300
Operating lease liability		$ 3,300