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, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenue
Contract revenue	$ 40,000								$ 40,000
Total revenue	40,000								40,000
Operating expenses
Research and development	1,317	$ 42,673	$ 28,618	$ 22,004	$ 16,731	$ 20,664	$ 16,397	$ 18,192	94,612	$ 71,984	$ 60,025
General and administrative	7,569	7,072	7,572	7,656	5,760	6,752	5,909	5,915	29,870	24,336	22,483
Total operating expenses	8,886	49,745	36,190	29,660	22,491	27,416	22,306	24,107	124,482	96,320	82,508
Income (loss) from operations	31,114	(49,745)	(36,190)	(29,660)	(22,491)	(27,416)	(22,306)	(24,107)	(84,482)	(96,320)	(82,508)
Interest income	186	220	242	199	160	120	129	105	847	514	218
Other income (expense), net	(547)	(262)	(521)	(261)	446	(39)	59	(230)	(1,591)	236	113
Net loss before provision for income taxes	30,753	(49,787)	(36,469)	(29,722)	(21,885)	(27,335)	(22,118)	(24,232)	(85,226)	(95,570)	(82,177)
Provision for (benefit from) income taxes	(375)		128		247				(247)	247
Net loss	$ 31,128	$ (49,787)	$ (36,597)	$ (29,722)					(84,979)	(95,817)	(82,177)
Net income (loss) per share- basic	$ 0.87	$ (1.40)	$ (1.04)	$ (0.85)
Weighted-average common shares used to compute basic net income (loss) per share	35,872	35,680	35,316	35,004
Net loss per share- diluted	$ 0.87	$ (1.40)	$ (1.04)	$ (0.85)
Weighted-average common shares used to compute diluted net income (loss) per share	35,901	35,680	35,316	35,004
Net loss attributable to common stockholders					$ (22,132)	$ (27,335)	$ (22,118)	$ (24,232)	$ (84,979)	$ (95,817)	$ (82,177)
Net loss per share- basic and diluted					$ (0.64)	$ (0.92)	$ (0.75)	$ (0.82)	$ (2.41)	$ (3.11)	$ (2.84)
Weighted-average common shares used to compute basic and diluted net loss per share					34,609	29,574	29,489	29,422	35,228	30,784	28,964