Quarterly Results (Unaudited) - Schedule of Quarterly Financial Information (Detail) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended			37 Months Ended	73 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2014
Operating expenses
Research and development	$ 11,602	$ 10,515	$ 5,622	$ 4,869	$ 3,454	$ 4,576	$ 3,326	$ 3,499	$ 32,608	$ 14,855	$ 10,963		$ 78,481
General and administrative	4,338	3,577	2,877	2,714	2,405	685	656	682	13,505	4,428	1,936		23,646
Total operating expenses	15,940	14,092	8,499	7,583	5,859	5,261	3,982	4,181	46,113	19,283	12,899		102,127
Loss from operations	(15,940)	(14,092)	(8,499)	(7,583)	(5,859)	(5,261)	(3,982)	(4,181)	(46,113)	(19,283)	(12,899)		(102,127)
Interest income	43	50	40		1				132	1			135
Interest expense								(128)		(128)	(393)		(863)
Other income (expense), net	244	208	(141)	(11,843)	44	252	327	290	(11,532)	913	75		(9,502)
Net loss and comprehensive loss	(15,653)	(13,834)	(8,600)	(19,426)	(5,814)	(5,009)	(3,655)	(4,019)	(57,513)	(18,497)	(13,217)	(23,130)	(112,357)
Deemed dividend related to beneficial conversion feature of convertible preferred stock				(25,559)					(25,559)				(25,559)
Net loss attributable to common stockholders	$ (15,653)	$ (13,834)	$ (8,600)	$ (44,985)	$ (5,814)	$ (5,009)	$ (3,655)	$ (4,019)	$ (83,072)	$ (18,497)	$ (13,217)		$ (136,818)
Net loss per basic and diluted share attributable to common stockholders	$ (0.65)	$ (0.57)	$ (0.36)	$ (16.13)	$ (4.62)	$ (19.98)	$ (24.74)	$ (30.23)	$ (4.39)	$ (41.10)	$ (114.71)
Weighted-average common shares used to compute basic and diluted net loss per share	24,215,018	24,194,808	24,194,808	2,788,087	1,257,250	250,745	147,713	132,969	18,921,533	450,000	115,219