Stock-based compensation (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense	$ 2,223	$ 2,288	$ 3,989	$ 5,326
Shares issued	0	1,836		22,132
Cash received from exercise of options				$ 200
Research and development expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense	$ 938	$ 1,478	1,919	2,936
General and administrative expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense	$ 1,285	$ 810	$ 2,070	$ 2,390