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Fair Value Measurement - Summary of Fair Value Assets and Liabilities (Details) - USD ($) $ in Thousands | Jun. 30, 2019 | Dec. 31, 2018
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Trade-in Guarantee |  |
Liabilities: |  |
Liabilities |  | $ 268
Money Market Funds |  |
Assets: |  |
Assets | $ 13,250 | 11,006
Level 1 | Money Market Funds |  |
Assets: |  |
Assets | $ 13,250 | 11,006
Level 3 | Trade-in Guarantee |  |
Liabilities: |  |
Liabilities |  | $ 268

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