ORGANIZATION, BASIS OF PRESENTATION, AND SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Additional Information (Details) $ in Thousands			3 Months Ended								12 Months Ended
	Feb. 05, 2020 shares	Aug. 09, 2017 USD ($)	Dec. 31, 2019 USD ($) shares	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($) shares	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Feb. 04, 2020 shares	Dec. 31, 2017 USD ($)
Class of Stock [Line Items]
Common stock, authorized (in shares) | shares			40,000,000				40,000,000				40,000,000	40,000,000
Accumulated deficit			$ 533,025				$ 420,548				$ 533,025	$ 420,548
License fee revenue			5,133	$ 103	$ 1,391	$ 516	$ 312	$ 292	$ 362	$ 154	7,143	1,120
Impairment of intangible assets			2,100								2,064	0
Cancer Prevention and Research Institute of Texas
Class of Stock [Line Items]
Grants awarded		$ 16,900
Revenue-sharing payments, maximum percent		1.00%
Deferred grant revenue														$ 4,200
License fee revenue
Class of Stock [Line Items]
Upfront license fee payments			5,000
License fee revenue			5,000								5,000	0
Grants
Class of Stock [Line Items]
License fee revenue											2,143	1,120
Grants | Cancer Prevention and Research Institute of Texas
Class of Stock [Line Items]
License fee revenue											2,100	$ 1,100
Maximum | Cancer Prevention and Research Institute of Texas
Class of Stock [Line Items]
Revenue-sharing payments, maximum percent		400.00%
Subsequent Event
Class of Stock [Line Items]
Reverse stock split ratio	0.1
Common stock, authorized (in shares) | shares	40,000,000												200,000,000
Held-for-sale
Class of Stock [Line Items]
Assets held for sale, property and equipment			12,000								12,000
Assets held for sale, right-of-use assets			4,800								4,800
Assets held for sale, current lease liabilities			1,500								1,500
Assets held for sale, long-term lease liabilities			$ 4,800								$ 4,800