Selected quarterly financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								6 Months Ended	12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 4,937	$ 4,937	$ 4,884	$ 2,576						$ 17,334
Total operating expenses	12,377	8,956	8,851	7,404	$ 4,496	$ 3,656	$ 3,230	$ 2,985	$ 3,766	37,588	$ 14,367
Loss from operations	(7,440)	(4,019)	(3,967)	(4,828)	(4,496)	(3,656)	(3,230)	(2,985)	(3,766)	(20,254)	(14,367)
Net loss attributable to common stockholders	$ (8,817)	$ (6,914)	$ (6,746)	$ (15,813)	$ (5,197)	$ (4,411)	$ (4,864)	$ (3,211)	$ (3,833)	$ (38,290)	$ (17,683)
Net loss per share attributable to common stock, basic and diluted	$ (0.67)	$ (5.25)	$ (5.80)	$ (15.81)	$ (6.58)	$ (6.45)	$ (8.11)	$ (6.93)	$ (1,629.68)	$ (9.14)	$ (27.83)