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STOCKHOLDERS' EQUITY - Stock grant agreements with non-employees, Stock (Details)
$ / shares in Units, $ in Thousands
1 Months Ended 3 Months Ended 9 Months Ended 12 Months Ended
Aug. 15, 2015
individual
Aug. 31, 2019
shares
Sep. 30, 2018
shares
Aug. 31, 2018
shares
Aug. 31, 2016
USD ($)
individual
$ / shares
shares
Aug. 31, 2015
individual
shares
Sep. 30, 2019
shares
Sep. 30, 2018
shares
Sep. 30, 2019
individual
shares
Dec. 31, 2020
individual
shares
Dec. 31, 2018
individual
shares
Dec. 31, 2017
shares
Dec. 31, 2016
shares
Dec. 31, 2015
shares
Nonemployees | Researchers, Employees of MSK                            
Share-Based Compensation Awards                            
Shares agreed to be issued           2,800,000                
Number of individuals | individual 2       2 2     1   2      
Shares repurchased and retired         83,600                  
Estimated fair value of stock repurchased and retired (in dollars per share) | $ / shares         $ 4.38                  
Reduction in equity | $         $ 366                  
Nonemployees | Researchers, Employees of MSK | Forecast                            
Share-Based Compensation Awards                            
Number of individuals | individual                   1        
Common Stock                            
Share-Based Compensation Awards                            
Issuance of common stock to nonemployees (in shares)             400,000 544,000            
Common Stock | 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
Common Stock | Nonemployees | Researchers, Employees of MSK | Forecast                            
Share-Based Compensation Awards                            
Issuance of common stock to nonemployees (in shares)                   400,000        
Common Stock | Nonemployees | Individual with stock grant agreement with issuance at IPO                            
Share-Based Compensation Awards                            
Issuance of common stock to nonemployees (in shares)     96,000