Collaboration arrangements and concentration of credit risk - Revenues from and amounts owed with collaboration partners (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total	$ 3,148	$ 2,260	$ 9,676	$ 10,523
Amounts owed by BMS in relation to the collaboration services	552		552		$ 1,586
BMS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total	3,148	$ 2,260	9,676	5,877
Chiesi Pharmaceutical
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total				$ 4,646
Before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total	$ 1,000		$ 3,200