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, 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
Operating expenses
Research and development	$ 16,731	$ 20,664	$ 16,397	$ 18,192	$ 15,586	$ 15,400	$ 11,940	$ 17,100	$ 71,984	$ 60,025	$ 32,608
General and administrative	5,760	6,752	5,909	5,915	4,622	5,124	7,556	5,181	24,336	22,483	13,505
Total operating expenses	22,491	27,416	22,306	24,107	20,208	20,524	19,496	22,281	96,320	82,508	46,113
Loss from operations	(22,491)	(27,416)	(22,306)	(24,107)	(20,208)	(20,524)	(19,496)	(22,281)	(96,320)	(82,508)	(46,113)
Interest income	160	120	129	105	51	54	65	49	514	218	132
Other income (expense), net	446	(39)	59	(230)	32	91	(236)	226	236	113	(11,532)
Net loss before provision for income taxes	(21,885)	(27,335)	(22,118)	(24,232)					(95,570)	(82,177)	(57,513)
Provision for income taxes	247								247
Net loss	$ (22,132)	$ (27,335)	$ (22,118)	$ (24,232)	(20,125)	(20,379)	(19,667)	(22,006)	(95,817)	(82,177)	(57,513)
Net loss attributable to common stockholders					$ (20,125)	$ (20,379)	$ (19,667)	$ (22,006)	$ (95,817)	$ (82,177)	$ (83,072)
Net loss per share- basic and diluted	$ (0.64)	$ (0.92)	$ (0.75)	$ (0.82)	$ (0.69)	$ (0.69)	$ (0.67)	$ (0.79)	$ (3.11)	$ (2.84)	$ (4.39)
Weighted-average common shares used to compute basic and diluted net loss per share	34,609	29,574	29,489	29,422	29,379	29,354	29,293	27,810	30,784	28,964	18,922