ORGANIZATION, BASIS OF PRESENTATION AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Narrative (Details) $ in Thousands				1 Months Ended	3 Months Ended		9 Months Ended	17 Months Ended
	Mar. 15, 2021 USD ($)	Apr. 14, 2020 USD ($)	Feb. 05, 2020 shares	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Dec. 31, 2020 USD ($) shares	Sep. 30, 2021 USD ($) segment subsidiary shares	Sep. 30, 2021 USD ($) shares	Mar. 26, 2021 USD ($)	Feb. 26, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 15, 2020 shares	Jun. 14, 2020 shares	Feb. 04, 2020 shares	Dec. 31, 2019 USD ($)
Class of Stock [Line Items]
Number of wholly-owned subsidiaries formed | subsidiary							2
Number of operating segments | segment							1
Number of reportable segments | segment							1
Reverse stock split ratio			0.1
Common stock, authorized (in shares) | shares			40,000,000			80,000,000	80,000,000	80,000,000				80,000,000	40,000,000	200,000,000
Accumulated deficit						$ (540,747)	$ (552,983)	$ (552,983)
Cash equivalents, restricted cash						36,996	20,767	20,767			$ 54,626				$ 93,816
Operating lease right-of-use assets						645	0	0
Property and equipment, net						189	27	27
Accrued clinical research costs						1,562	$ 1,658	$ 1,658
Scenario, Adjustment
Class of Stock [Line Items]
Accrued clinical research costs											1,200
Accrued manufacturing costs and accrued professional fees, current											$ 100
Maximum
Class of Stock [Line Items]
Escrow deposits related to property sales, period up to								18 months
2130 W. Holcombe Blvd.
Class of Stock [Line Items]
Escrow deposits cash proceeds received		$ 1,500
Research And Development Facility | Disposal Group, Held-for-sale, Not Discontinued Operations
Class of Stock [Line Items]
Operating lease right-of-use assets						500
Property and equipment, net						2,300
Operating lease, liability						700
Impairment charges						$ 1,300
Reed Road, Houston, TX
Class of Stock [Line Items]
Amount paid upon closing of asset sale										$ 1,100
Discontinued operation, consideration, cash										900
Discontinued operation, consideration, note receivable										$ 200
San Francisco Office Space
Class of Stock [Line Items]
Operating lease right-of-use assets									$ 600
Operating lease, liability									1,000
Lease termination fee	$ 900
Security deposit returned				$ 200
Loss on termination of lease					$ 500
San Francisco Office Space | Equipment
Class of Stock [Line Items]
Operating lease right-of-use assets									$ 100