Collaboration arrangements and concentration of credit risk - Revenues from and amounts owed with collaboration partners (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total	$ 3,478
Amounts owed by BMS in relation to the collaboration services	1,057		$ 1,586
BMS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total	$ 3,478
Before Topic 606
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total		$ 3,321
Before Topic 606 | BMS
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total		1,507
Before Topic 606 | Chiesi Pharmaceutical
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Total		$ 1,814