Annual report pursuant to Section 13 and 15(d)

STOCKHOLDERS' EQUITY - Common Stock (Details)

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STOCKHOLDERS' EQUITY - Common Stock (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 3 Months Ended 12 Months Ended
Aug. 31, 2020
Jun. 30, 2018
Dec. 31, 2020
Dec. 31, 2018
Dec. 31, 2019
May 18, 2018
May 17, 2018
Controlled Equity Offering              
Authorized number of shares of common stock     400,000,000   400,000,000    
Aggregate proceeds       $ 67,162      
Common stock              
Controlled Equity Offering              
Authorized number of shares of common stock           400,000,000 200,000,000
Number of shares of common stock sold (in shares)   18,400,000   18,400,000      
Share price (in dollars per share)   $ 3.90          
Common stock | Open Market Sales Agreement              
Controlled Equity Offering              
Number of shares of common stock sold (in shares)     0        
Common stock | Open Market Sales Agreement | Maximum              
Controlled Equity Offering              
Maximum value of shares available under Controlled Equity Offering $ 65,000