Stock-Based Compensation - Summary of Stock-Based Compensation Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	3,522,813
Number of Shares Outstanding, Ending balance	6,543,654	3,522,813
2008 Plan and 2015 Plan [Member]
Options Outstanding, Shares Available for Grant
Shares Available for Grant, Beginning balance	3,523,879	1,550,101	637,768
Shares Available for Grant, Awards authorized	1,202,324	3,400,000	3,221,477
Shares Available for Grant, Granted	(3,710,980)	(1,471,664)	(2,309,681)
Shares Available for Grant, Exercised	0	0	0
Shares Available for Grant, Cancelled	377,298	45,442	537
Shares Available for Grant, Ending balance	1,392,521	3,523,879	1,550,101	637,768
Shares Available for Grant, Awards authorized	1,202,324	3,400,000	3,221,477
Number of Shares Outstanding, Exercisable	2,614,587
Shares Available for Grant, Granted	(3,710,980)	(1,471,664)	(2,309,681)
Number of Shares Outstanding, Vested and expected to vest	6,352,640
Shares Available for Grant, Exercised	0	0	0
Shares Available for Grant, Cancelled	377,298	45,442	537
Shares Available for Grant, Ending balance	1,392,521	3,523,879	1,550,101	637,768
Options Outstanding, Number of Shares Outstanding
Number of Shares Outstanding, Beginning balance	3,522,813	2,138,096	582,152
Number of Shares Outstanding, Options granted	3,710,980	1,471,664	2,309,681
Number of Shares Outstanding, Options exercised	(312,841)	(41,505)	(752,698)
Number of Shares Outstanding, Options cancelled	(377,298)	(45,442)	(1,039)
Number of Shares Outstanding, Ending balance	6,543,654	3,522,813	2,138,096	582,152
Options Outstanding, Weighted-Average Exercise Price Per Share
Weighted-Average Exercise Price Per Share, Beginning balance	$ 3.81	$ 0.91	$ 0.37
Weighted-Average Exercise Price Per Share, Options granted	3.22	8.06	0.93
Weighted-Average Exercise Price Per Share, Options exercised	0.67	0.76	0.54
Weighted-Average Exercise Price Per Share, Options cancelled	7.04	7.76	2.08
Weighted-Average Exercise Price Per Share, Ending balance	3.44	$ 3.81	$ 0.91	$ 0.37
Weighted-Average Exercise Price Per Share, Exercisable	2.41
Weighted-Average Exercise Price Per Share, Vested and expected to vest	$ 3.43
Options Outstanding, Weighted-Average Remaining Contractual Term (Years)
Weighted-Average Remaining Contractual Term (Years)	8 years 9 months 26 days	8 years 9 months 11 days	9 years 3 months 15 days	8 years 7 months 17 days
Weighted-Average Remaining Contractual Term (Year), Exercisable	7 years 11 months 27 days
Weighted-Average Remaining Contractual Term (Year), Vested and expected to vest	8 years 9 months 22 days
Options Outstanding, Aggregate Intrinsic Value of Outstanding Options
Aggregate Intrinsic Value of Outstanding Options	$ 888	$ 40,425	$ 3,153	$ 48
Aggregate Intrinsic Value of Outstanding Options, Exercisable	888
Aggregate Intrinsic Value of Outstanding Options, Vested and expected to vest	$ 877