Stock-based compensation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense	$ 2,760	$ 2,804	$ 8,086	$ 7,912
Shares issued		7,294	22,132	34,210
Cash received from exercise of options		$ 200	$ 200	$ 500
Research and development expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense	1,361	959	4,297	2,698
General and administrative expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense	$ 1,399	$ 1,845	$ 3,789	$ 5,214