STOCKHOLDERS' EQUITY - Issuance of common stock (Details) $ / shares in Units, $ in Thousands	1 Months Ended					3 Months Ended				9 Months Ended	12 Months Ended
	Aug. 31, 2020 shares	Jul. 31, 2020 individual	Apr. 30, 2020 individual shares	Nov. 30, 2019 USD ($) $ / shares shares	Aug. 31, 2015 individual	Sep. 30, 2020 shares	Jun. 30, 2020 individual shares	Mar. 31, 2020 shares	Sep. 30, 2019 shares	Sep. 30, 2020 shares	Dec. 31, 2019 individual shares	Dec. 31, 2018 individual shares	Dec. 31, 2017 shares	Dec. 31, 2016 shares	Dec. 31, 2015 shares
Issuance of stock
Exercise of stock options (in shares)										83,694
SADA License Agreement
Issuance of stock
Issuance of common stock to strategic partner (in shares)							42,900
Researchers, Employees of MSK | SADA License Agreement
Issuance of stock
Number of individuals | individual		2
Researchers, Employees of MSK | Nonemployees
Issuance of stock
Number of individuals | individual					2						1	2
Researchers, Employees of MSK | Nonemployees | SADA License Agreement
Issuance of stock
Number of individuals | individual			2				2
Secondary Public Offering
Issuance of stock
Payment of offering costs | $				$ 9,100
Common Stock
Issuance of stock
Exercise of stock options (in shares)						57,916		25,778
Restricted stock awards (in shares)								3,429
Issuance of common stock to non-employees (in shares)						400,000	256,896		400,000
Common Stock | Researchers, Employees of MSK | Nonemployees
Issuance of stock
Issuance of common stock to non-employees (in shares)	400,000										400,000	544,000	448,000	448,000	560,000
Common Stock | Researchers, Employees of MSK | Nonemployees | SADA License Agreement
Issuance of stock
Issuance of common stock to non-employees (in shares)			213,996				213,996
Common Stock | Secondary Public Offering
Issuance of stock
Shares issued (in shares)				5,134,750
Share price (in dollars per share) | $ / shares				$ 28.00
Issuance of shares of common stock, net of issuance costs | $				$ 134,704