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STOCKHOLDERS' EQUITY - Stock grant agreements with non-employees, Stock (Details) $ in Thousands	1 Months Ended				3 Months Ended			9 Months Ended	12 Months Ended
	Aug. 31, 2020 shares	Jul. 31, 2020 USD ($) individual	Apr. 30, 2020 USD ($) individual shares	Aug. 31, 2015 individual shares	Sep. 30, 2020 shares	Jun. 30, 2020 USD ($) individual shares	Sep. 30, 2019 shares	Sep. 30, 2020 USD ($)	Dec. 31, 2019 individual shares	Dec. 31, 2018 individual item shares	Dec. 31, 2017 shares	Dec. 31, 2016 shares	Dec. 31, 2015 shares
Share-Based Compensation Awards
Non-cash expense in connection with equity issuance to inventors \| $								$ 7,376
Nonemployees \| Researchers, Employees of MSK
Share-Based Compensation Awards
Number of individuals \| individual				2					1	2
Number of stock grants fully issued \| item										1
SADA License Agreement \| Researchers, Employees of MSK
Share-Based Compensation Awards
Number of individuals \| individual		2
Notes receivable, noncurrent \| $		$ 2,610
Notes receivable term		3 years
Notes receivable interest rate (as a percent)		1.00%
SADA License Agreement \| Nonemployees \| Researchers, Employees of MSK
Share-Based Compensation Awards
Number of individuals \| individual			2			2
Non-cash expense in connection with equity issuance to inventors \| $			$ 7,376			$ 7,376
SADA License Agreement \| Nonemployees \| Researchers, Employees of MSK \| Tranche one
Share-Based Compensation Awards
Vesting (as a percent)			40.00%
SADA License Agreement \| Nonemployees \| Researchers, Employees of MSK \| Tranche two
Share-Based Compensation Awards
Vesting (as a percent)			60.00%
Vesting period			3 years
Common Stock
Share-Based Compensation Awards
Issuance of common stock to non-employees (in shares)					400,000	256,896	400,000
Common Stock \| Nonemployees \| Researchers, Employees of MSK
Share-Based Compensation Awards
Shares agreed to be issued				2,800,000
Issuance of common stock to non-employees (in shares)	400,000								400,000	544,000	448,000	448,000	560,000
Common Stock \| SADA License Agreement \| Nonemployees \| Researchers, Employees of MSK
Share-Based Compensation Awards
Shares agreed to be issued			213,996
Issuance of common stock to non-employees (in shares)			213,996			213,996