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Common Stock and Warrants - Summary of Warrants Accounted for as Liabilities and Recorded as Derivative Financial Instruments (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Fair Value Disclosures [Abstract] |  |
Fair value of warrants at beginning of year | $ 6,941 | $ 2,857
Issuance of warrants | 7,372 | 3,016
Exercise of warrants | (3,012) | (1,200)
Fair value of previously classified equity warrants | 5,049 |
Fair value of previously classified liability warrants reclassified to additional paid-in capital | (2,478) |
Foreign exchange gains | (872) |
Change in fair value of warrants during the year | 769 | 2,268
Fair value of warrants at end of year | $ 13,769 | $ 6,941

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