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Significant Accounting Policies - Assets and Liability Measured at Fair Value on Recurring Basis (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Asset: |  |
Asset, fair value | $ 529,539 |
Money Market Fund |  |
Asset: |  |
Asset, fair value | 156,655 | $ 10,078
U.S. Treasury Shares |  |
Asset: |  |
Asset, fair value | 48,219 |
Corporate Bonds |  |
Asset: |  |
Asset, fair value | 147,472 |
Commercial Paper |  |
Asset: |  |
Asset, fair value | 135,173 |
Agency Securities |  |
Asset: |  |
Asset, fair value | 19,999 |
Repo Securities |  |
Asset: |  |
Asset, fair value | 12,000 |
Asset Backed Securities |  |
Asset: |  |
Asset, fair value | 10,021 |
Level 1 |  |
Asset: |  |
Asset, fair value | 204,874 |
Level 1 | Money Market Fund |  |
Asset: |  |
Asset, fair value | 156,655 | $ 10,078
Level 1 | U.S. Treasury Shares |  |
Asset: |  |
Asset, fair value | 48,219 |
Level 2 |  |
Asset: |  |
Asset, fair value | 324,665 |
Level 2 | Corporate Bonds |  |
Asset: |  |
Asset, fair value | 147,472 |
Level 2 | Commercial Paper |  |
Asset: |  |
Asset, fair value | 135,173 |
Level 2 | Agency Securities |  |
Asset: |  |
Asset, fair value | 19,999 |
Level 2 | Repo Securities |  |
Asset: |  |
Asset, fair value | 12,000 |
Level 2 | Asset Backed Securities |  |
Asset: |  |
Asset, fair value | $ 10,021 |

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