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Summary of Significant Accounting Policies - Short-Term Investments (Details) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
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Schedule of Held-to-maturity Securities [Line Items] |  |
Short-term investments | $ 8,505 | $ 8,504
Cash and Fixed Income Securities |  |
Schedule of Held-to-maturity Securities [Line Items] |  |
Short-term investments | $ 8,500 | $ 8,500

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