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Marketable Securities - Schedule of Amortized Cost, Gross Unrealized Holding Losses and Fair Value of Available-for-Sale Debt Securities by Type of Security (Detail) - USD ($) | Sep. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Available for sale securities: |  |
Fair Value |  | $ 0
U.S. Government-Related Obligations [Member] |  |
Available for sale securities: |  |
Amortized Cost | $ 312,476,000 |
Gross Unrealized Holding Gains | 122,000 |
Fair Value | $ 312,598,000 |

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