Stock-based compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended		6 Months Ended
	Feb. 28, 2015	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Issuance of common stock, net of costs (in shares)	2,587,500
Share price of stock issued (in US$ per share)	$ 20.00
Proceeds from sale of common stock, net	$ 48,400			$ 48,361	$ 0
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense		$ 2,792	$ 1,285	$ 5,108	2,231
Shares issued		4,750		26,916
Cash received from exercise of options		$ 100		$ 300
Research and development expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense		931	582	1,739	1,070
General and administrative expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense		$ 1,861	$ 703	$ 3,369	$ 1,161