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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 |  |

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