Stock Option Activity under Stock Option Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Number of Shares
Beginning Balance	3,852,135
Granted	506,000
Exercised	(104,167)
Cancelled	(184,500)
Ending Balance	4,069,468	3,852,135
Options exercisable, at end of period	2,924,501	2,925,502
Options available for future grant	5,053,295
Weighted Average Exercise Price
Beginning Balance	$ 5.12
Granted	3.50
Exercised	2.30
Cancelled	5.77
Ending Balance	5.24	$ 5.12
Options exercisable, at end of period	$ 5.13	$ 5.12
Weighted Average Contractual Term (Years)
Outstanding, at end of period	6 years 2 months 1 day
Options exercisable, at end of period	4 years 11 months 23 days	5 years 6 months 29 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 576
Options exercisable, at end of period	$ 498	$ 1,152

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.