Stock-Based Compensation - Summary of Stock-Based Compensation Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	6,543,654
Number of Shares Outstanding, Ending balance	7,470,601	6,543,654
2008 Plan and 2015 Plan [Member]
Options Outstanding, Shares Available for Grant
Shares Available for Grant, Beginning balance	1,392,521	3,523,879	1,550,101
Shares Available for Grant, Awards authorized	1,214,837	1,202,324	3,400,000
Shares Available for Grant, Granted	(1,502,025)	(3,710,980)	(1,471,664)
Shares Available for Grant, Exercised	0	0	0
Shares Available for Grant, Cancelled	398,437	377,298	45,442
Shares Available for Grant, Ending balance	1,503,770	1,392,521	3,523,879	1,550,101
Shares Available for Grant, Awards authorized	1,214,837	1,202,324	3,400,000
Number of Shares Outstanding, Exercisable	3,995,823
Shares Available for Grant, Granted	(1,502,025)	(3,710,980)	(1,471,664)
Number of Shares Outstanding, Vested and expected to vest	7,470,601
Shares Available for Grant, Exercised	0	0	0
Shares Available for Grant, Cancelled	398,437	377,298	45,442
Shares Available for Grant, Ending balance	1,503,770	1,392,521	3,523,879	1,550,101
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	6,543,654	3,522,813	2,138,096
Number of Shares Outstanding, Options granted	1,502,025	3,710,980	1,471,664
Number of Shares Outstanding, Options exercised	(176,641)	(312,841)	(41,505)
Number of Shares Outstanding, Options cancelled	(398,437)	(377,298)	(45,442)
Number of Shares Outstanding, Ending balance	7,470,601	6,543,654	3,522,813	2,138,096
Options Outstanding, Weighted-Average Exercise Price Per Share
Weighted-Average Exercise Price Per Share, Beginning balance	$ 3.44	$ 3.81	$ 0.91
Weighted-Average Exercise Price Per Share, Options granted	1.45	3.22	8.06
Weighted-Average Exercise Price Per Share, Options exercised	0.94	0.67	0.76
Weighted-Average Exercise Price Per Share, Options cancelled	3.55	7.04	7.76
Weighted-Average Exercise Price Per Share, Ending balance	3.09	$ 3.44	$ 3.81	$ 0.91
Weighted-Average Exercise Price Per Share, Exercisable	3.10
Weighted-Average Exercise Price Per Share, Vested and expected to vest	$ 3.09
Options Outstanding, Weighted-Average Remaining Contractual Term (Years)
Weighted-Average Remaining Contractual Term (Years)	8 years 1 month 13 days	8 years 9 months 25 days	8 years 9 months 11 days	9 years 3 months 15 days
Weighted-Average Remaining Contractual Term (Year), Exercisable	7 years 7 months 10 days
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest	8 years 1 month 13 days
Options Outstanding, Aggregate Intrinsic Value of Outstanding Options
Aggregate Intrinsic Value of Outstanding Options	$ 12,363	$ 888	$ 40,425	$ 3,153
Aggregate Intrinsic Value of Outstanding Options, Exercisable	6,749
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest	$ 12,363