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Summary of Significant Accounting Policies - Additional Information (Details)
 3 Months Ended 12 Months Ended
Mar. 31, 2019
USD ($)
$ / shares
shares
Mar. 31, 2018
shares
Dec. 31, 2018
USD ($)
Segment
$ / shares
shares
Dec. 31, 2017
USD ($)
Segment
shares
Jul. 25, 2018
USD ($)
Dec. 31, 2016
USD ($)
Summary Of Significant Accounting Policies [Line Items]            
Cash equivalents investments with original maturities     $ 0 $ 0    
Impairment loss     0 0    
Goodwill $ 372,000   372,000 372,000    
Qualifying expenditures related to research     2,700,000 3,500,000    
Interest or penalties related to income tax matters     $ 0 $ 0    
Number of operating segment | Segment     1 1    
Dilutive securities excluded from the calculations of diluted loss per share (in shares) | shares 300,000 46,858 300,000 9,458    
Restricted cash $ 40,000   $ 40,000 $ 675,000   $ 400,000
Cash, cash equivalents $ 3,872,000   $ 5,261,000 9,550,000   $ 12,600,000
Outstanding Warrants [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Dilutive securities excluded from the calculations of diluted loss per share (in shares) | shares 200,000   200,000      
Options and Restricted Stock [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Dilutive securities excluded from the calculations of diluted loss per share (in shares) | shares     2,753      
Preferred Stock and Restricted Stock Awards [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Dilutive securities excluded from the calculations of diluted loss per share (in shares) | shares 92,523   94,589      
Customer Concentration Risk [Member] | Accounts Receivable [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Concentration risk percentage 100.00% 100.00%        
Barda Contract | Development            
Summary Of Significant Accounting Policies [Line Items]            
Revenue recognized     $ 3,000,000 3,700,000    
2018 Rights Offering [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Redemption price of warrant prior to expiration | $ / shares $ 0.50   $ 0.50      
Common stock price per share for warrant redemption | $ / shares $ 181.50   $ 181.50      
Number of consecutive trading days for warrant redemption 20 days   20 days      
Warrants liability fair value $ 1,500,000   $ 900,000   $ 3,100,000  
Barda Contract            
Summary Of Significant Accounting Policies [Line Items]            
Goodwill     $ 0 $ 0    
Barda Contract | Customer Concentration Risk [Member] | Revenue, Product and Service Benchmark [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Concentration risk percentage     100.00% 100.00%    
Barda Contract | Customer Concentration Risk [Member] | Accounts Receivable [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Concentration risk percentage     100.00% 100.00%    
Minimum [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Useful lives of the assets     3 years      
Maximum [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Useful lives of the assets     5 years      
Letter of Credit [Member]            
Summary Of Significant Accounting Policies [Line Items]            
Letters of credit amount pursuant to lease agreement     $ 40,000 $ 700,000