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Organization and Operations - Liquidity and Going Concern (Details) - USD ($)
$ / shares in Units, $ in Thousands
 1 Months Ended 12 Months Ended
Jan. 01, 2018
Aug. 31, 2016
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Oct. 31, 2017
Mar. 31, 2017
Dec. 31, 2014
Common Stock                
Share price (in dollars per share)     $ 0.001          
Additional common stock issued     $ 8,036 $ 11,994 $ 61,744      
Proceeds from registered direct offering, net of issuance costs       10,603        
Accumulated deficit     (171,434) (144,070)        
Cash and cash equivalents     $ 16,369 $ 23,596 $ 62,780     $ 14,668
Common Stock                
Common Stock                
Number of shares issued     4,804,796 3,243,015 5,750,000      
Additional common stock issued     $ 5 $ 3 $ 5      
Common Stock | Cowen                
Common Stock                
Percentage of commission payable to underwriter based on proceeds from common stock   3.00%            
Common Stock | First ATM Program                
Common Stock                
Number of shares issued     4,804,796          
Share price (in dollars per share)     $ 1.79          
Additional common stock issued     $ 8,600          
Net proceeds     8,000          
Common Stock | First ATM Program | Cowen                
Common Stock                
Aggregate maximum offering amount   $ 10,000         $ 8,800  
Common Stock | Second ATM Program                
Common Stock                
Remaining common stock available for sale under offering program     $ 8,600          
Common Stock | Second ATM Program | Cowen                
Common Stock                
Aggregate maximum offering amount           $ 10,000    
Common Stock | Second ATM Program | Subsequent Events                
Common Stock                
Number of shares issued 5,390,255              
Share price (in dollars per share) $ 1.59              
Additional common stock issued $ 8,600              
Net proceeds $ 8,300              
Common Stock | Second ATM Program | Subsequent Events | Cowen                
Common Stock                
Number of shares issued 5,390,255              
Share price (in dollars per share) $ 1.59              
Additional common stock issued $ 8,600              
Net proceeds $ 8,300              
Common Stock | Registered Direct Offering                
Common Stock                
Number of shares issued       3,243,015        
Share price (in dollars per share)       $ 4.04        
Additional common stock issued       $ 13,100        
Proceeds from registered direct offering, net of issuance costs       $ 12,000