Stock-based compensation (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense	$ 1,479	$ 2,760	$ 5,468	$ 8,086
Shares issued	7,937	0	0	22,132
Proceeds from stock options exercised		$ 40		$ 200
Research and development expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense	$ 584	1,361	$ 2,504	4,297
General and administrative expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense	$ 895	$ 1,399	$ 2,964	$ 3,789