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Significant Accounting Policies - Estimated Fair Value of Assets and Liability (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Asset: |  |
Asset, fair value | $ 493,278 |
Money Market Fund |  |
Asset: |  |
Asset, fair value | 81,714 | $ 10,078
U.S. Treasury Shares |  |
Asset: |  |
Asset, fair value | 59,959 |
Corporate Bonds |  |
Asset: |  |
Asset, fair value | 200,704 |
Commercial Paper |  |
Asset: |  |
Asset, fair value | 110,858 |
Agency Securities |  |
Asset: |  |
Asset, fair value | 14,027 |
Repo Securities |  |
Asset: |  |
Asset, fair value | 11,000 |
Asset Backed Securities |  |
Asset: |  |
Asset, fair value | 8,887 |
Certificate of Deposit |  |
Asset: |  |
Asset, fair value | 6,129 |
Level 1 |  |
Asset: |  |
Asset, fair value | 141,673 |
Level 1 | Money Market Fund |  |
Asset: |  |
Asset, fair value | 81,714 | $ 10,078
Level 1 | U.S. Treasury Shares |  |
Asset: |  |
Asset, fair value | 59,959 |
Level 2 |  |
Asset: |  |
Asset, fair value | 351,605 |
Level 2 | Corporate Bonds |  |
Asset: |  |
Asset, fair value | 200,704 |
Level 2 | Commercial Paper |  |
Asset: |  |
Asset, fair value | 110,858 |
Level 2 | Agency Securities |  |
Asset: |  |
Asset, fair value | 14,027 |
Level 2 | Repo Securities |  |
Asset: |  |
Asset, fair value | 11,000 |
Level 2 | Asset Backed Securities |  |
Asset: |  |
Asset, fair value | 8,887 |
Level 2 | Certificate of Deposit |  |
Asset: |  |
Asset, fair value | $ 6,129 |

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