STOCKHOLDERS' EQUITY - Issuance of common stock, Stock grant agreements (Details) - Common Stock - shares	1 Months Ended			3 Months Ended		9 Months Ended	12 Months Ended
	Aug. 31, 2019	Sep. 30, 2018	Aug. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-Based Compensation Awards
Issuance of common stock to nonemployees (in shares)				400,000	544,000
Nonemployees | Individual with stock grant agreement with issuance at IPO
Share-Based Compensation Awards
Issuance of common stock to nonemployees (in shares)		96,000
Nonemployees | Researchers, Employees of MSK
Share-Based Compensation Awards
Issuance of common stock to nonemployees (in shares)	400,000		448,000			400,000	448,000	448,000	448,000	560,000