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Composition of Certain Financial Statement Captions - Accounts Payable and Accrued Expenses (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
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Accounts Payable and Accrued Expenses [Abstract] |  |  |
Accrued expenses |  | $ 824 | $ 1,182
Accounts payable |  | 721 | 1,297
Accrued payroll and bonus |  | 423 | 750
Accrued legal fees |  | 186 | 509
Accrued vacation |  | 192 | 64
Accrued R&D studies |  | 230 | 285
Other current liabilities |  | 201 | 119
Accounts payable and accrued expenses, Total | $ 2,729 | $ 2,777 | $ 4,206

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