STOCK-BASED COMPENSATION - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Unrecognized share-based compensation expense	$ 5.7
Unrecognized share-based compensation expense, period for recognition	3 years 3 months 25 days
Estimated forfeiture rate	7.00%
Options granted (in shares)	1,088,260
Weighted-average exercise price (in usd per share)	$ 4.67
Vical Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Expiration term	10 years
Maximum shares permitted to be granted in a single calendar year (in shares)	1,300,000
Options \| 2009 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized (in shares)	1,634,655
Shares available for grant (in shares)	59,011
Options \| Vical Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares authorized (in shares)	413,710
Shares available for grant (in shares)	86,584
Employee (including Board of Directors)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)	982,890	252,681
Weighted-average grant date fair value (in dollars per share)	$ 3.24	$ 4.16
Weighted-average exercise price (in usd per share)	$ 4.58	$ 5.68
Non-employee (excluding Board of Directors)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Options granted (in shares)	105,370	15,188
Weighted-average exercise price (in usd per share)	$ 5.53	$ 5.68

Share-based Compensation Arrangement By Share-based Payment Award, Number Of Shares Authorized For Grant In One Year

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- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Fair Value Assumptions, Expected Forfeiture Rate

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- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Number of shares authorized for issuance under share-based payment arrangement.

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- Definition

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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- Definition

Gross number of share options (or share units) granted during the period.

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- Definition

The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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- Definition

Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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