Annual report pursuant to Section 13 and 15(d)

STOCKHOLDERS' EQUITY - Equity Offerings (Details)

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STOCKHOLDERS' EQUITY - Equity Offerings (Details) - USD ($)
$ / shares in Units, $ in Thousands
1 Months Ended 8 Months Ended 12 Months Ended 17 Months Ended
Oct. 31, 2017
May 31, 2017
Feb. 28, 2017
Aug. 31, 2015
Dec. 31, 2017
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2016
Controlled Equity Offering                  
Aggregate proceeds           $ 114,180 $ 23,586 $ 5,292  
Common stock                  
Controlled Equity Offering                  
Number of shares of common stock sold (in shares) 20,815,000   23,000,000   2,166,093 45,981,093 7,895,563 1,722,312 9,617,875
Share price (in dollars per share) $ 3.35   $ 2.00            
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