Stock Option Activity Under Stock Option Plan (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Number of Shares
Beginning Balance	5,842,879
Granted	896,000	1,496,333
Exercised	(2,333)
Cancelled	(59,667)
Ending Balance	6,676,879
Options exercisable, at end of period	3,024,728		2,765,357
Options available for future grant	1,066,275
Weighted Average Exercise Price
Beginning Balance	$ 3.21
Granted	4.17
Exercised	1.87
Cancelled	4.67
Ending Balance	3.95
Options exercisable, at end of period	$ 4.05		$ 4.39
Weighted Average Contractual Term (Years)
Outstanding, at end of period	8 years 1 month 24 days
Options exercisable, at end of period	6 years 9 months 29 days	6 years 8 months 12 days
Aggregate Intrinsic Value
Outstanding, at end of period	$ 864
Options exercisable, at end of period	$ 509		$ 3,603

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.