Collaboration agreements - Merck KGaA (Details) - Merck KGaA $ in Thousands	1 Months Ended
	Jun. 30, 2014 USD ($) item	Sep. 30, 2019 USD ($) item	Dec. 31, 2017 item
Collaboration agreements
Upfront payment received	$ 12,000
Number of targets | item	6
Amount of additional fees receivable when target is designated and commercial license to target is granted	$ 0
Number of targets designated | item		6	6
Next potential development milestone payment eligible to receive		$ 500