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Significant Accounting Policies - Estimated Fair Value of Assets and Liability (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Asset: |  |
Asset, fair value | $ 473,882 |
Level 1 |  |
Asset: |  |
Asset, fair value | 136,622 |
Level 2 |  |
Asset: |  |
Asset, fair value | 337,260 |
Money Market Fund |  |
Asset: |  |
Asset, fair value | 88,266 | $ 10,078
Money Market Fund | Level 1 |  |
Asset: |  |
Asset, fair value | 88,266 | $ 10,078
U.S. Treasury Shares |  |
Asset: |  |
Asset, fair value | 48,356 |
U.S. Treasury Shares | Level 1 |  |
Asset: |  |
Asset, fair value | 48,356 |
Corporate Bonds |  |
Asset: |  |
Asset, fair value | 179,864 |
Corporate Bonds | Level 2 |  |
Asset: |  |
Asset, fair value | 179,864 |
Commercial Paper |  |
Asset: |  |
Asset, fair value | 106,650 |
Commercial Paper | Level 2 |  |
Asset: |  |
Asset, fair value | 106,650 |
Agency Securities |  |
Asset: |  |
Asset, fair value | 20,782 |
Agency Securities | Level 2 |  |
Asset: |  |
Asset, fair value | 20,782 |
Repo Securities |  |
Asset: |  |
Asset, fair value | 10,000 |
Repo Securities | Level 2 |  |
Asset: |  |
Asset, fair value | 10,000 |
Asset Backed Securities |  |
Asset: |  |
Asset, fair value | 8,599 |
Asset Backed Securities | Level 2 |  |
Asset: |  |
Asset, fair value | 8,599 |
Certificate of Deposit |  |
Asset: |  |
Asset, fair value | 6,125 |
Certificate of Deposit | Level 2 |  |
Asset: |  |
Asset, fair value | 6,125 |
Non-US Government |  |
Asset: |  |
Asset, fair value | 5,240 |
Non-US Government | Level 2 |  |
Asset: |  |
Asset, fair value | $ 5,240 |

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