Stockholders' Equity - Common Stock and Warrants (Details) - USD ($)
 1 Months Ended 6 Months Ended 12 Months Ended
Apr. 12, 2017
Apr. 03, 2017
Dec. 22, 2016
Feb. 28, 2018
Apr. 30, 2017
Dec. 31, 2016
Jan. 31, 2014
Feb. 28, 2013
Jun. 30, 2018
Jun. 30, 2017
Dec. 31, 2017
Jul. 20, 2017
Stockholders' equity                        
Preferred stock, Shares authorized                 5,000,000      
Preferred stock, par value                 $ 0.01      
Common Stock, par value (in dollars per share)                 $ 0.01   $ 0.01  
Common Stock Shares, Outstanding                 214,653,162   228,486,366  
Exercise price of warrants (in dollars per share)                       $ 2.1231
Proceeds from exercise of warrants, net of transaction costs                   $ 17,702,000    
Series A Junior Participating Cumulative Preferred Stock                        
Stockholders' equity                        
Preferred stock, par value                 $ 0.01      
Common Stock Shares, Outstanding                 0   0  
At Market Issuance Sales Agreement                        
Stockholders' equity                        
Common Stock, par value (in dollars per share)   $ 0.01                    
Authorized amount   $ 75,000,000                    
Shares issued   $ 23,000,000                    
Warrants issued in February, 2013                        
Stockholders' equity                        
Class of Warrant or Right Issued               23,637,500        
Exercise price of warrants (in dollars per share)       $ 0.15       $ 0.15        
Warrants outstanding (in shares)                 0   100  
Number of warrants exercised (in shares)       100                
Warrants issued in December 2016                        
Stockholders' equity                        
Class of Warrant or Right Issued     10,501,500     10,501,500            
Exercise price of warrants (in dollars per share)     $ 1.50     $ 1.50            
Number of warrants exercised (in shares) 10,501,500                   10,501,500  
Proceeds from exercise of warrants, net of transaction costs $ 15.10                      
Shares of common stock that can be purchased from warrants issued 5,250,750                      
Per share price of shares of common stock $ 2.69                      
Warrants issued in January, 2014                        
Stockholders' equity                        
Class of Warrant or Right Issued         4,000,000   4,000,000          
Exercise price of warrants (in dollars per share)         $ 0.65 $ 0.65 $ 4.00          
Number of warrants exercised (in shares)                 4,000,000      
Proceeds from exercise of warrants, net of transaction costs         $ 2,600,000              
Increase in additional paid-in capital         27,100,000              
Reduction of warrant liability         $ 27,100,000              
Warrant Issued With Amazon And Walmart Stores Inc Transaction Agreement In 2017                        
Stockholders' equity                        
Class of Warrant or Right Issued                     110,573,392  
Number of warrants exercised (in shares)                 18,913,869   18,913,869