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SUBSEQUENT EVENT (Details) - USD ($)
 1 Months Ended 12 Months Ended
Jan. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Subsequent Event [Line Items]        
Issuance of common stock in connection with at market offering, net of issuance costs   $ 19,295,000 $ 86,017,000 $ 1,060,000
2018 Sales Agreement        
Subsequent Event [Line Items]        
Issuance of common stock in connection with at market offering, net of issuance costs (in shares)   13,214,867 10,083,079  
Issuance of common stock in connection with at market offering, net of issuance costs   $ 19,295,000 $ 38,366,000  
Subsequent Event | 2018 Sales Agreement        
Subsequent Event [Line Items]        
Issuance of common stock in connection with at market offering, net of issuance costs (in shares) 530,228      
Issuance of common stock in connection with at market offering, net of issuance costs $ 748,000