Stock-Based Compensation - Summary of Stock-Based Compensation Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance		6,543,654
Number of Shares Outstanding, Ending balance	7,685,449	7,685,449	6,543,654
2008 Plan and 2015 Plan [Member]
Options Outstanding, Shares Available for Grant
Shares Available for Grant, Beginning balance		1,392,521
Shares Available for Grant, Awards authorized		1,214,837
Shares Available for Grant, Granted		(1,444,425)
Shares Available for Grant, Exercised		0
Shares Available for Grant, forfeited/cancelled		252,769
Shares Available for Grant, Ending balance	1,415,702	1,415,702	1,392,521
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance		6,543,654
Number of Shares Outstanding, Options granted		1,444,425
Number of Shares Outstanding, Options exercised	0	(49,861)
Number of Shares Outstanding, Options forfeited/cancelled		(252,769)
Number of Shares Outstanding, Ending balance	7,685,449	7,685,449	6,543,654
Number of Shares Outstanding, Exercisable	3,796,870	3,796,870
Number of Shares Outstanding, Vested and expected to vest	7,685,449	7,685,449
Options Outstanding, Weighted-Average Exercise Price Per Share
Weighted-Average Exercise Price Per Share, Beginning balance		$ 3.44
Weighted-Average Exercise Price Per Share, Options granted		1.43
Weighted-Average Exercise Price Per Share, Options exercised		0.46
Weighted-Average Exercise Price Per Share, Options forfeited/cancelled		3.85
Weighted-Average Exercise Price Per Share, Ending balance	$ 3.07	3.07	$ 3.44
Weighted-Average Exercise Price Per Share, Exercisable	2.97	2.97
Weighted-Average Exercise Price Per Share, Vested and expected to vest	$ 3.07	$ 3.07
Options Outstanding, Weighted-Average Remaining Contractual Term (Years)
Weighted-Average Remaining Contractual Term (Years)		8 years 4 months 6 days	8 years 9 months 25 days
Weighted-Average Remaining Contractual Term (Year), Exercisable		7 years 9 months 3 days
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest		8 years 4 months 6 days
Options Outstanding, Aggregate Intrinsic Value of Outstanding Options
Aggregate Intrinsic Value of Outstanding Options	$ 1,100	$ 1,100	$ 888
Aggregate Intrinsic Value of Outstanding Options, Exercisable	942	942
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest	$ 1,100	$ 1,100