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Significant Accounting Policies - Summary of Derivative Financial Instruments (Details) - Fair Value, Measurements, Recurring - Black Scholes Model - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Liabilities: |  |
Fair value, liabilities |  | $ 9,578
Warrants |  |
Liabilities: |  |
Fair value, liabilities | $ 13,769 | 6,941
Non-employee Stock Options |  |
Liabilities: |  |
Fair value, liabilities |  | 2,637
Non-current Receivable |  |
Assets: |  |
Fair value, assets | 294 |
Level 3 |  |
Liabilities: |  |
Fair value, liabilities |  | 9,578
Level 3 | Warrants |  |
Liabilities: |  |
Fair value, liabilities | 13,769 | 6,941
Level 3 | Non-employee Stock Options |  |
Liabilities: |  |
Fair value, liabilities |  | $ 2,637
Level 3 | Non-current Receivable |  |
Assets: |  |
Fair value, assets | $ 294 |

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