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Composition of Certain Financial Statement Captions - Other Current Assets (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | --- | ---
Other Current Assets [Abstract] |  |  |
Prepaid services, current |  | $ 166 | $ 121
Prepaid insurance |  | 564 | 556
Other receivables |  | 55 | 103
Total other current assets | $ 736 | $ 785 | $ 780

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