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Significant Accounting Policies - Estimated Fair Value of Assets and Liability (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands | Sep. 30, 2021 | Dec. 31, 2020
--- | --- | ---
Asset: |  |
Asset, fair value | $ 420,014 | $ 473,882
Money Market Fund |  |
Asset: |  |
Asset, fair value | 116,456 | 88,266
U.S. Treasury Shares |  |
Asset: |  |
Asset, fair value | 22,214 | 48,356
Corporate Bonds |  |
Asset: |  |
Asset, fair value | 95,771 | 179,864
Commercial Paper |  |
Asset: |  |
Asset, fair value | 126,832 | 106,650
Agency Securities |  |
Asset: |  |
Asset, fair value | 6,748 | 20,782
Repo Securities |  |
Asset: |  |
Asset, fair value | 9,000 | 10,000
Asset Backed Securities |  |
Asset: |  |
Asset, fair value | 9,895 | 8,599
Certificates of Deposit |  |
Asset: |  |
Asset, fair value | 21,575 | 6,125
Non-US Government |  |
Asset: |  |
Asset, fair value | 11,523 | 5,240
Level 1 |  |
Asset: |  |
Asset, fair value | 138,670 | 136,622
Level 1 | Money Market Fund |  |
Asset: |  |
Asset, fair value | 116,456 | 88,266
Level 1 | U.S. Treasury Shares |  |
Asset: |  |
Asset, fair value | 22,214 | 48,356
Level 2 |  |
Asset: |  |
Asset, fair value | 281,344 | 337,260
Level 2 | Corporate Bonds |  |
Asset: |  |
Asset, fair value | 95,771 | 179,864
Level 2 | Commercial Paper |  |
Asset: |  |
Asset, fair value | 126,832 | 106,650
Level 2 | Agency Securities |  |
Asset: |  |
Asset, fair value | 6,748 | 20,782
Level 2 | Repo Securities |  |
Asset: |  |
Asset, fair value | 9,000 | 10,000
Level 2 | Asset Backed Securities |  |
Asset: |  |
Asset, fair value | 9,895 | 8,599
Level 2 | Certificates of Deposit |  |
Asset: |  |
Asset, fair value | 21,575 | 6,125
Level 2 | Non-US Government |  |
Asset: |  |
Asset, fair value | $ 11,523 | $ 5,240

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