Selected quarterly financial data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 2,362	$ 3,308	$ 3,720	$ 4,830	$ 4,937	$ 4,937	$ 4,884	$ 2,576	$ 14,220	$ 17,334
Total operating expenses	16,205	13,679	13,338	12,297	12,377	8,956	8,851	7,404	55,519	37,588	$ 14,367
Loss from operations	(13,843)	(10,371)	(9,618)	(7,467)	(7,440)	(4,019)	(3,967)	(4,828)	(41,299)	(20,254)	(14,367)
Net loss attributable to common stockholders	$ (14,674)	$ (8,996)	$ (9,335)	$ (7,188)	$ (8,817)	$ (6,914)	$ (6,746)	$ (15,813)	$ (40,193)	$ (38,290)	$ (17,683)
Net loss per share attributable to common stock, basic and diluted	$ (0.57)	$ (0.35)	$ (0.37)	$ (0.29)	$ (0.67)	$ (5.25)	$ (5.80)	$ (15.81)	$ (1.59)	$ (9.14)	$ (27.83)