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 8 Months Ended 12 Months Ended 17 Months Ended
Oct. 31, 2017
May 31, 2017
Feb. 28, 2017
Aug. 31, 2015
Jun. 30, 2018
Dec. 31, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2016
May 18, 2018
May 17, 2018
Controlled Equity Offering                        
Authorized number of shares of common stock           400,000,000 400,000,000 400,000,000        
Aggregate proceeds             $ 67,162 $ 114,180 $ 23,586      
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) 20,815,000   23,000,000   18,400,000 2,166,093 18,400,000 45,981,093 7,895,563 9,617,875    
Share price (in dollars per share) $ 3.35   $ 2.00   $ 3.90              
Aggregate proceeds $ 65,300   $ 43,000     $ 5,700       $ 30,000    
Value of common stock shares registered for sale                 $ 0 $ 0    
Cantor | Common stock                        
Controlled Equity Offering                        
Maximum value of shares available under Controlled Equity Offering   $ 40,000   $ 30,000