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Stockholders' Equity - Common Stock (Details) - USD ($) |  |  |  |  |  |  | 3 Months Ended | 7 Months Ended | 10 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 21, 2018 | Jul. 25, 2018 | Jun. 01, 2018 | Nov. 28, 2017 | May 31, 2017 | Apr. 11, 2017 | Mar. 31, 2019 | Dec. 31, 2018 | Mar. 31, 2018 | Dec. 31, 2018 | Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock issued (in shares) |  | 6,723 |  | 10,000 |  |  |  |  |  |  |  |  |
Proceeds from sale of common stock, net |  |  |  |  |  |  | $ 1,919,000 |  | $ (150,000) |  |  | $ 0 | $ 0
Common stock issued, value |  |  |  |  |  |  | $ 1,873,000 |  | $ 27,000 |  |  | $ 1,624,000 | $ 12,716,000
Common Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock issued (in shares) |  | 6,723 |  | 10,000 |  |  | 139,855 |  | 201 |  |  | 92,169 | 24,474
Share issued, price per share |  |  |  |  |  | $ 550.00 |  |  |  |  |  |  |
Common Stock [Member] | Maxim Group LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock issued (in shares) |  |  |  |  | 1,698 | 17,881 |  |  |  |  |  |  |
Share issued, price per share |  |  |  |  | $ 550.00 | $ 520.00 |  |  |  |  |  |  |
Proceeds from sale of common stock, net |  |  |  |  | $ 800,000 | $ 8,700,000 |  |  |  |  |  |  |
Closing of offering date |  |  |  |  |  | Apr. 17, 2017 |  |  |  |  |  |  |
Number of days granted as option to purchase additional shares of common stock to underwriter |  |  |  |  |  | 45 days |  |  |  |  |  |  |
Additional shares of common stock |  |  |  |  |  | 1,888 |  |  |  |  |  |  |
Common Stock [Member] | B. Riley FBR [Member] | Sales Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock issued (in shares) |  |  |  |  |  |  |  |  |  | 1,584 | 200,000 |  |
Proceeds from sale of common stock, net |  |  |  |  |  |  |  |  |  | $ 1,700,000 | $ 3,800,000 |  |
Common Stock [Member] | B. Riley FBR [Member] | Sales Agreement [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of common stock, net |  |  | $ 6,500,000 |  |  |  |  |  |  |  |  |  |
Common Stock [Member] | B. Riley FBR [Member] | ATM Program [Member] | Sales Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock issued (in shares) |  |  |  |  |  |  |  |  |  | 79,234 | 200,000 |  |
Proceeds from sale of common stock, net |  |  |  |  |  |  |  |  |  | $ 1,700,000 |  |  |
Proceeds from sale of common stock, net |  |  |  |  |  |  | $ 3,800,000 |  |  |  | $ 3,800,000 |  |
Common Stock [Member] | B. Riley FBR [Member] | ATM Program [Member] | Sales Agreement [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from sale of common stock, net |  |  | $ 6,500,000 |  |  |  |  |  |  |  |  |  |
Common Stock [Member] | Lincoln Park Capital Fund, LLC [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock issued, value | $ 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Period exercisable from the date of issuance | 24 months |  |  |  |  |  |  |  |  |  |  |  |
Common stock issued (in shares) |  |  |  |  |  |  | 0 |  |  |  |  |  |
Common Stock [Member] | Lincoln Park Capital Fund, LLC [Member] | Securities Purchase Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Trading Volume of Common Shares | 0 |  |  |  |  |  |  |  |  |  |  |  |
Upper Limit on the Price Per Share | $ 0 |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Member] | Lincoln Park Capital Fund, LLC [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Beneficial ownership percentage of common stock outstanding | 4.99% |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Member] | Lincoln Park Capital Fund, LLC [Member] | Maximum [Member] | Single Regular Purchase [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock issued (in shares) | 5,000 |  |  |  |  |  |  |  |  |  |  |  |
Common stock issued (in shares) | 5,000 |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Member] | Lincoln Park Capital Fund, LLC [Member] | Minimum [Member] | Single Regular Purchase [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock issued, value | $ 1,000,000 |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Member] | Lincoln Park Capital Fund, LLC [Member] | Purchase Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock issued (in shares) |  |  |  |  |  |  |  | 12,802 |  |  |  |  |
Proceeds from sale of common stock, net |  |  |  |  |  |  |  | $ 300,000 |  |  |  |  |
Common stock issued (in shares) |  |  |  |  |  |  |  | 12,802 |  |  |  |  |
Common Stock [Member] | Lincoln Park Capital Fund, LLC [Member] | Purchase Agreement [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Common Stock [Abstract] |  |  |  |  |  |  |  |  |  |  |  |  |
Floor price of per share | $ 12.50 |  |  |  |  |  |  |  |  |  |  |  |

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