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Fair Value of Financial Assets - Summary of Financial Assets Measured at Fair Value on a Recurring Basis and Level of the Fair Value Hierarchy (Detail) - Fair value on a recurring basis [Member] - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Investments: |  |
Total | $ 677,561 | $ 355,520
Investments [Member] |  |
Investments: |  |
Total investments | 230,415 | 313,862
Investments [Member] | US treasury bills [Member] |  |
Investments: |  |
Total investments | 33,026 | 232,604
Investments [Member] | US agency securities [Member] |  |
Investments: |  |
Total investments | 197,389 | 81,258
Level 1 [Member] |  |
Investments: |  |
Total | 447,146 | 41,658
Level 2 [Member] |  |
Investments: |  |
Total | 230,415 | 313,862
Level 2 [Member] | Investments [Member] |  |
Investments: |  |
Total investments | 230,415 | 313,862
Level 2 [Member] | Investments [Member] | US treasury bills [Member] |  |
Investments: |  |
Total investments | 33,026 | 232,604
Level 2 [Member] | Investments [Member] | US agency securities [Member] |  |
Investments: |  |
Total investments | 197,389 | 81,258
Cash equivalents [Member] | Money market funds [Member] |  |
Cash equivalents: |  |
Assets, fair value | 447,146 | 41,658
Cash equivalents [Member] | Level 1 [Member] | Money market funds [Member] |  |
Cash equivalents: |  |
Assets, fair value | $ 447,146 | $ 41,658

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