Stock-based compensation (Details) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended
	Sep. 30, 2015	Feb. 28, 2015	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Issuance of common stock, net of costs (in shares)	2,250,000	2,587,500
Share price of stock issued (in US$ per share)	$ 45.00	$ 20.00	$ 45.00		$ 45.00
Proceeds from sale of common stock, net	$ 94,900	$ 48,400			$ 143,289	$ 0
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense			$ 2,804	$ 2,591	$ 7,912	4,822
Shares issued			7,294		34,210
Cash received from exercise of options			$ 200		$ 500
Research and development expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense			959	859	2,698	1,929
General and administrative expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense			$ 1,845	$ 1,732	$ 5,214	$ 2,893