Quarterly Results (Unaudited) - Schedule of Quarterly Financial Information (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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
Operating expenses
Research and development	$ 15,586	$ 15,400	$ 11,940	$ 17,100	$ 11,602		$ 10,515		$ 5,622		$ 4,869		$ 60,025	$ 32,608	$ 14,855
General and administrative	4,622	5,124	7,556	5,181	4,338		3,577		2,877		2,714		22,483	13,505	4,428
Total operating expenses	20,208	20,524	19,496	22,281	15,940		14,092		8,499		7,583		82,508	46,113	19,283
Loss from operations	(20,208)	(20,524)	(19,496)	(22,281)	(15,940)		(14,092)		(8,499)		(7,583)		(82,508)	(46,113)	(19,283)
Interest income	51	54	65	49	43		50		40				218	132	1
Interest expense															(128)
Other income (expense), net	32	91	(236)	226	244	[1]	208	[1]	(141)	[1]	(11,843)	[1]	113	(11,532)	913
Net loss and comprehensive loss	(20,125)	(20,379)	(19,667)	(22,006)	(15,653)		(13,834)		(8,600)		(19,426)		(82,177)	(57,513)	(18,497)
Deemed dividend related to beneficial conversion feature of convertible preferred stock											(25,559)	[2]		(25,559)
Net loss attributable to common stockholders	$ (20,125)	$ (20,379)	$ (19,667)	$ (22,006)	$ (15,653)		$ (13,834)		$ (8,600)		$ (44,985)		$ (82,177)	$ (83,072)	$ (18,497)
Net loss per basic and diluted share attributable to common stockholders	$ (0.69)	$ (0.69)	$ (0.67)	$ (0.79)	$ (0.65)		$ (0.57)		$ (0.36)		$ (16.13)		$ (2.84)	$ (4.39)	$ (41.10)
Weighted-average common shares used to compute basic and diluted net loss per share	29,379	29,354	29,293	27,810	24,215		24,195		24,195		2,788		28,964	18,922	450

[1] The first quarter of 2014 includes a change in the fair value of the preferred stock call option liability associated with the Series D convertible preferred stock financing of approximately $9.6 million and a $2.3 million change in the fair value of the warrant liability associated with the Series B convertible preferred stock.

[2] The first quarter of 2014 also includes a deemed dividend of approximately $25.6 million, which reflected a beneficial conversion feature on the underlying Series E preferred stock, in connection with the closing of the IPO on March 26, 2014.