Condensed Quarterly Financial Data (unaudited) - Schedule of Condensed Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 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, 2015	Dec. 31, 2014	Dec. 31, 2013
Quarterly Financial Information Disclosure [Abstract]
Total revenue	$ 22,315	$ 15,693	$ 13,066	$ 11,593	$ 15,618	$ 12,343	$ 10,881	$ 8,751	$ 62,667	$ 47,593	$ 31,403
Net loss	$ (8,816)	$ (9,467)	$ (12,404)	$ (14,894)	$ (12,380)	$ (12,148)	$ (14,088)	$ (11,422)	$ (45,581)	$ (50,038)	$ (29,281)
Net loss per share - basic and diluted	$ (0.44)	$ (0.49)	$ (0.66)	$ (0.81)	$ (0.68)	$ (0.67)	$ (0.78)	$ (0.68)	$ (2.40)	$ (2.80)	$ (4.44)