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Summary of Significant Accounting Policies - Schedule of Compensating Balance Arrangements (Detail) - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020 | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | --- | --- | ---
Cash and Cash Equivalents [Line Items] |  |  |  |
Total restricted cash | $ 1,327 | $ 1,730 | $ 1,905 | $ 1,902
Restricted Cash | 1,327 | 1,994 |  |
Restricted cash, current portion | 0 | 264 | $ 0 | $ 0
Letter of Credit | Cambridge Lease Agreement |  |  |  |
Cash and Cash Equivalents [Line Items] |  |  |  |
Restricted Cash | 0 | 264 |  |
Letter of Credit | Waltham Lease |  |  |  |
Cash and Cash Equivalents [Line Items] |  |  |  |
Restricted Cash | 250 | 250 |  |
Letter of Credit | Vienna Austria Lease |  |  |  |
Cash and Cash Equivalents [Line Items] |  |  |  |
Restricted Cash | 222 | 336 |  |
Letter of Credit | Allston Lease Agreement |  |  |  |
Cash and Cash Equivalents [Line Items] |  |  |  |
Restricted Cash | $ 855 | $ 1,144 |  |

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