SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Merger Narrative (Details) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Aug. 31, 2019 USD ($) shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2019 USD ($) segment shares	Dec. 31, 2018 USD ($)	Jun. 02, 2018 $ / shares shares
Business Acquisition [Line Items]
Number of operating segments | segment			1
Proceeds from the exercise of warrants | $			$ 47	$ 0
Vical Warrants
Business Acquisition [Line Items]
Warrants converted (in shares) | shares					891,582
Exercise price (in usd per share) | $ / shares					$ 0.07
Warrants exercised (in shares) | shares		670,288	670,288
Proceeds from the exercise of warrants | $		$ 47
Vical
Business Acquisition [Line Items]
Common stock issued in recapitalization (in shares) | shares	3,367,988
Net acquired tangible assets | $	$ 36,093