Stock-Based Compensation - Summary of Stock-Based Compensation Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
2008 Plan, 2015 Plan and 2018 Plan [Member]
Options Outstanding, Shares Available for Grant
Shares Available for Grant, Beginning balance	1,503,770
Shares Available for Grant, Granted	(3,486,150)
Shares Available for Grant, Exercised	0
Shares Available for Grant, forfeited/cancelled	118,072
Shares Available for Grant, Ending balance	1,731,500	1,503,770
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	7,470,601
Number of Shares Outstanding, Options granted	3,486,150
Number of Shares Outstanding, Options exercised	(118,942)
Number of Shares Outstanding, Options forfeited/cancelled	(118,072)
Number of Shares Outstanding, Ending balance	10,719,737	7,470,601
Number of Shares Outstanding, Exercisable	5,376,902
Number of Shares Outstanding, Vested and expected to vest	10,719,737
Options Outstanding, Weighted-Average Exercise Price Per Share
Weighted-Average Exercise Price Per Share, Beginning balance	$ 3.09
Weighted-Average Exercise Price Per Share, Options granted	2.37
Weighted-Average Exercise Price Per Share, Options exercised	1.50
Weighted-Average Exercise Price Per Share, Options forfeited/cancelled	4.31
Weighted-Average Exercise Price Per Share, Ending balance	2.86	$ 3.09
Weighted-Average Exercise Price Per Share, Exercisable	3.07
Weighted-Average Exercise Price Per Share, Vested and expected to vest	$ 2.86
Options Outstanding, Weighted-Average Remaining Contractual Term (Years)
Weighted-Average Remaining Contractual Term (Years)	8 years 21 days	8 years 1 month 13 days
Weighted-Average Remaining Contractual Term (Year), Exercisable	7 years 2 months 8 days
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest	8 years 21 days
Options Outstanding, Aggregate Intrinsic Value of Outstanding Options
Aggregate Intrinsic Value of Outstanding Options	$ 1,420	$ 12,363
Aggregate Intrinsic Value of Outstanding Options, Exercisable	1,251
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest	$ 1,420
2015 Plan [Member]
Options Outstanding, Shares Available for Grant
Shares Available for Grant, Awards authorized	2,095,808
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	7,470,601
Number of Shares Outstanding, Ending balance	10,719,737	7,470,601
2018 Equity Inducement Plan [Member]
Options Outstanding, Shares Available for Grant
Shares Available for Grant, Awards authorized	1,500,000
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Options granted	0
Number of Shares Outstanding, Ending balance	0