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Summary of Significant Accounting Policies - Summary of Derivative Financial Instruments for Non-employee Stock Options (Details) - Fair Value, Measurements, Recurring - Black Scholes Model - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Non-employee Stock Options |  |
Liabilities: |  |
Fair value, liabilities | $ 1,329 | $ 2,637
Warrants |  |
Liabilities: |  |
Fair value, liabilities | 2,478 | 6,941
Level 3 | Non-employee Stock Options |  |
Liabilities: |  |
Fair value, liabilities | 1,329 | 2,637
Level 3 | Warrants |  |
Liabilities: |  |
Fair value, liabilities | $ 2,478 | $ 6,941

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- Definition Fair value of financial and nonfinancial obligations. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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