Stock-based compensation (Details) (USD $)	1 Months Ended	3 Months Ended
	Feb. 28, 2015	Mar. 31, 2015	Mar. 31, 2014
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]
Issuance of common stock, net of costs (in shares)	2,587,500
Share price of stock issued (in US$ per share)	$ 20.00
Proceeds from sale of common stock, net	$ 48,400,000	$ 48,361,000	$ 0
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense		2,316,000	946,000
Shares issued		22,166	23,307
Cash received from exercise of options		300,000
Research and development expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense		808,000	488,000
General and administrative expense
Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items]
Allocated share-based compensation expense		$ 1,508,000	$ 458,000