Equity Incentive Plans - Summary of Activity Under 2008 Plan, 2014 Plan and Inducement Plan Including Grants to Nonemployees Issued (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Shares Available for Grant, Beginning balance	1,027,237	1,634,420	1,265,282
Shares Available for Grant, Options authorized	1,892,376	1,038,595	743,569
Shares Available for Grant, Options granted	(1,723,906)	(1,524,014)	(379,709)
Shares Available for Grant, Options exercised	0	0	0
Shares Available for Grant, Options canceled	445,029	67,743	28,563
Issuance of common stock for services and restricted stock units, net of forfeitures	601,008	189,507	23,285
Shares Available for Grant, Issuance of common stock for services	(46,858)	(20,118)	(13,285)
Shares Available for Grant, Ending balance	1,039,728	1,027,237	1,634,420
Options Issued and Outstanding Number of Shares, Beginning balance	2,712,223	1,281,086	1,007,724
Options Issued and Outstanding Number of Shares, Options authorized	0	0	0
Options Issued and Outstanding Number of Shares, Options granted	1,723,906	1,524,014	379,709
Options Issued and Outstanding Number of Shares, Options exercised	(35,759)	(25,134)	(77,784)
Options Issued and Outstanding Number, Options cancelled	(445,029)	(67,743)	(28,563)
Options Issued and Outstanding Number of Shares, Issuance of common stock for services	0	0	0
Options Issued and Outstanding Number of Shares, Ending balance	3,955,341	2,712,223	1,281,086
Options Issued and Outstanding Average Exercise Price per Share, Beginning balance	$ 10.60	$ 9.78	$ 5.51
Options Issued and Outstanding Number of Shares, Expected to vest	3,840,963
Options Issued and Outstanding Number of Shares, Exercisable	1,978,711
Options Issued and Outstanding Average Exercise Price per Share, Options granted	$ 11.73	11.42	19.60
Options Issued and Outstanding Average Exercise Price per Share, Options exercised	1.70	2.19	1.42
Options Issued and Outstanding Average Exercise Price per Share, Options canceled	14.15	16.67	12.41
Options Issued and Outstanding Average Exercise Price per Share, Issuance of common stock for services	0	0	0
Options Issued and Outstanding Average Exercise Price per Share, Ending balance	10.78	$ 10.60	$ 9.78
Options Issued and Outstanding Average Exercise Price per Share, Expected to vest	10.74
Options Issued and Outstanding Average Exercise Price per Share, Exercisable	$ 9.74
Weighted Average Remaining Contractual Term, Ending Balance	7 years 8 months 1 day
Weighted Average Remaining Contractual Term, Vested and Expected to Vest	7 years 7 months 17 days
Weighted Average Remaining Contractual Term, Exercisable	6 years 6 months 18 days
Aggregate Intrinsic Value, Ending balance	$ 4,393
Aggregate Intrinsic Value, Expected to Vest	4,363
Aggregate Intrinsic Value, Exercisable	$ 3,767
Share available for grant, forfeitures	79,850	5,000