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Stockholders' Equity - Stock and Warrant Issuances - Additional Information (Detail)
 1 Months Ended 6 Months Ended 12 Months Ended
Oct. 11, 2017
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Oct. 10, 2017
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Apr. 02, 2017
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Nov. 30, 2016
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Investor
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Oct. 31, 2016
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Jun. 30, 2016
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Jul. 31, 2015
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Feb. 28, 2015
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Oct. 31, 2013
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Jun. 30, 2018
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$ / shares
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Jun. 30, 2017
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Dec. 31, 2017
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Investor
$ / shares
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Dec. 31, 2016
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$ / shares
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Dec. 31, 2015
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$ / shares
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Nov. 30, 2017
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Jun. 06, 2016
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Oct. 20, 2015
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Dec. 31, 2014
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Stockholders Equity Note [Line Items]                                    
Sale of common stock, net of expenses (in shares)         2   3,000,000 1 1 424,000                
Proceeds from issuance of private placement | $         $ 1,200,000     $ 2,600,000 $ 7,500,000                  
Net proceeds from issuance of private placement | $         $ 1,100,000     $ 2,500,000 $ 6,900,000 $ 4,300,000                
Number of warrants outstanding (in shares)                       14,049 78 44       9
Sale of units to underwriter             3,000,000                      
Proceeds of common stock issued, net of related expenses | $             $ 6,000,000                      
Number of pre-funded warrants                   76,000                
Maximum [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants exercisable per share price (in dollars per share) | $ / shares                       $ 19,712,000 $ 19,712,000 $ 19,712,000       $ 19,712,000
Maximum offering and selling of stock and securities that entity make to raise capital | $                                 $ 71,800,000  
Common Stock Issued [Member]                                    
Stockholders Equity Note [Line Items]                                    
Sale of common stock, net of expenses (in shares)                   668,854     2 4        
Common shares and rights issued                       227,287 10          
Amendment to Restructuring Agreement [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants issued to purchase Common Stock 127,551 225                   255            
Warrants exercisable per share price (in dollars per share) | $ / shares                       $ 61,250            
Number of warrants outstanding (in shares)                       10            
Warrants term                       4 years 6 months            
Warrants exercised (in shares)                       3            
Number of investors | Investor                       2            
Shares issuable upon warrants exercised                       735            
Warrants exercise price description                       The warrants contain a cashless exercise provision that allows the holders to exercise each warrant for three shares of common stock.            
Securities Purchase Agreement [Member] | Common Stock Issued [Member]                                    
Stockholders Equity Note [Line Items]                                    
Common shares and rights issued                       224,000            
Securities Purchase Agreement [Member] | Warrant Repurchase Agreements [Member]                                    
Stockholders Equity Note [Line Items]                                    
Aggregate amount to be paid in exchange for cancellation of warrants | $     $ 7,900,000     $ 7,900,000                        
Senior Secured Convertible Notes [Member] | Securities Purchase Agreement [Member]                                    
Stockholders Equity Note [Line Items]                                    
Debt instrument, principal face amount | $                               $ 35,000,000    
Senior Secured Convertible Notes [Member] | Securities Purchase Agreement [Member] | Common Stock Issued [Member]                                    
Stockholders Equity Note [Line Items]                                    
Common shares and rights issued                       217,800            
Series A Common Stock Warrants [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants exercisable per share price (in dollars per share) | $ / shares                       $ 24,500            
Number of warrants outstanding (in shares)                       9,000,000            
Number of warrants converted             8,000,000                      
Gross proceeds from underwriting offering | $             $ 7,000,000                      
Warrants exercised (in shares)                       1,000,000            
Net proceeds from exercise of warrants | $                       $ 400,000            
Series A And B Common Stock Warrants [Member]                                    
Stockholders Equity Note [Line Items]                                    
Value of warrants issued | $             $ 3,400,000                      
Series B Common Stock Warrants [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants exercised (in shares)                         1,000,000          
Net proceeds from exercise of warrants | $                         $ 800,000          
Remaining outstanding warrants expired                         3,000,000          
Warrants expiration date                       Jan. 29, 2016            
Expected Life [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants term                   5 years                
Expected Life [Member] | Maximum [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants term                   6 years 4 months 24 days   4 years 10 months 17 days            
'Expected Volatility [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants measurement input                   1.9410                
'Expected Volatility [Member] | Maximum [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants measurement input                   3.7200   2.6692            
Risk-free Interest Rates [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants measurement input                   0.0278                
Risk-free Interest Rates [Member] | Maximum [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants measurement input                   0.0292   0.0206            
February 2018 Warrants [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants issued to purchase Common Stock                   1,000,000                
Value of warrants issued | $                   $ 18,300,000                
Warrants exercisable per share price (in dollars per share) | $ / shares                   $ 10.00                
Number of warrants outstanding (in shares)                   1,000,000                
February 2018 Warrants [Member] | Exercise Price [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants measurement input | $ / shares                   10.00                
February 2018 Warrants [Member] | Expected Life [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants term                   6 years                
February 2018 Warrants [Member] | 'Expected Volatility [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants measurement input                   1.2268                
February 2018 Warrants [Member] | Risk-free Interest Rates [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants measurement input                   0.01                
2013 Warrants [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants issued to purchase Common Stock                 9                  
Value of warrants issued | $                 $ 1,900,000                  
Warrants exercisable per share price (in dollars per share) | $ / shares                       $ 19,712,000            
Number of warrants outstanding (in shares)                       9            
Warrants term                       5 years            
February 2015 Warrants [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants issued to purchase Common Stock               9                    
Value of warrants issued | $               $ 800,000                    
Warrants exercisable per share price (in dollars per share) | $ / shares                       $ 24,500            
Number of warrants outstanding (in shares)                       9            
Warrants term                       5 years            
Series C Warrants [Member]                                    
Stockholders Equity Note [Line Items]                                    
Aggregate amount to be paid in exchange for cancellation of warrants | $                   $ 0 $ 7,876,000 $ 7,876,000 $ 0 $ 0        
Series C Warrants [Member] | Securities Purchase Agreement [Member]                                    
Stockholders Equity Note [Line Items]                                    
Value of warrants issued | $           27,800,000                        
Series C Warrants [Member] | Securities Purchase Agreement [Member] | Warrant Repurchase Agreements [Member]                                    
Stockholders Equity Note [Line Items]                                    
Aggregate amount to be paid in exchange for cancellation of warrants | $           7,900,000                        
Series C Warrants [Member] | Senior Secured Convertible Notes [Member]                                    
Stockholders Equity Note [Line Items]                                    
Debt instrument, principal face amount | $           $ 35,000,000                        
Series C Warrants [Member] | Senior Secured Convertible Notes [Member] | Securities Purchase Agreement [Member]                                    
Stockholders Equity Note [Line Items]                                    
Number of warrants converted           40                        
Shares issuable upon warrants exercised                               40    
October 2016 Warrants [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants issued to purchase Common Stock         1                          
Value of warrants issued | $         $ 300,000                          
Warrants exercisable per share price (in dollars per share) | $ / shares                       $ 24,500            
Number of warrants outstanding (in shares)                       1            
Warrants term                       5 years            
Net proceeds from exercise of warrants | $                       $ 100,000            
November 2017 Warrants [Member] | Exchange Agreement [Member]                                    
Stockholders Equity Note [Line Items]                                    
Warrants issued to purchase Common Stock       14,049                            
Value of warrants issued | $       $ 14,400,000                     $ 14,400,000      
Warrants exercisable per share price (in dollars per share) | $ / shares                       $ 1,225            
Number of warrants outstanding (in shares)                       14,049            
Warrants term                       5 years            
Number of investors | Investor       2