| Fair Value Measurement - Summary of Fair Value Assets and Liabilities (Details) - USD ($) $ in Thousands | Sep. 30, 2019 | Dec. 31, 2018 | 
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| Trade-in Guarantee | ||
| Liabilities: | ||
| Liabilities | $ 268 | |
| Money Market Funds | ||
| Assets: | ||
| Assets | $ 14,050 | 11,006 | 
| Level 1 | Money Market Funds | ||
| Assets: | ||
| Assets | $ 14,050 | 11,006 | 
| Level 3 | Trade-in Guarantee | ||
| Liabilities: | ||
| Liabilities | $ 268 | 
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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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- Definition Fair value of financial and nonfinancial obligations. Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef 
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