STOCKHOLDERS' EQUITY - Issuance of common stock (Details) $ / shares in Units, $ in Thousands	1 Months Ended				3 Months Ended		6 Months Ended	12 Months Ended
	Apr. 30, 2020 individual shares	Jan. 31, 2020 shares	Nov. 30, 2019 USD ($) $ / shares shares	Aug. 31, 2015 individual	Jun. 30, 2020 USD ($) shares	Mar. 31, 2020 shares	Jun. 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
Issuance of shares of common stock, net of issuance costs | $					$ 8,707
Exercise of stock options (in shares)							25,778
SADA License Agreement
Issuance of stock
Issuance of common stock to strategic partner (in shares)	42,900				42,900
Nonemployees | 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
Secondary Public Offering
Issuance of stock
Payment of offering costs | $			$ 9,100
Common Stock
Issuance of stock
Shares issued (in shares)					256,896
Exercise of stock options (in shares)						25,778
Restricted stock awards (in shares)						3,429
Common Stock | Director
Issuance of stock
Exercise of stock options (in shares)		25,778
Common Stock | Employee
Issuance of stock
Restricted stock awards (in shares)		3,429
Common Stock | Researchers, Employees of MSK | Nonemployees
Issuance of stock
Issuance of common stock to nonemployees (in shares)								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 nonemployees (in shares)	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