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Significant Accounting Policies - Estimated Fair Value of Assets and Liability (Detail) - Fair Value, Measurements, Recurring - USD ($) $ in Thousands | Dec. 31, 2021 | Dec. 31, 2020
--- | --- | ---
Asset: |  |
Asset, fair value | $ 374,911 | $ 473,882
Level 1 |  |
Asset: |  |
Asset, fair value | 118,282 | 136,622
Level 2 |  |
Asset: |  |
Asset, fair value | 256,629 | 337,260
Money Market Fund |  |
Asset: |  |
Asset, fair value | 95,063 | 88,266
Money Market Fund | Level 1 |  |
Asset: |  |
Asset, fair value | 95,063 | 88,266
U.S. Treasury Shares |  |
Asset: |  |
Asset, fair value | 23,219 | 48,356
U.S. Treasury Shares | Level 1 |  |
Asset: |  |
Asset, fair value | 23,219 | 48,356
Corporate Bonds |  |
Asset: |  |
Asset, fair value | 83,587 | 179,864
Corporate Bonds | Level 2 |  |
Asset: |  |
Asset, fair value | 83,587 | 179,864
Commercial Paper |  |
Asset: |  |
Asset, fair value | 121,773 | 106,650
Commercial Paper | Level 2 |  |
Asset: |  |
Asset, fair value | 121,773 | 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 | 8,000 | 10,000
Repo Securities | Level 2 |  |
Asset: |  |
Asset, fair value | 8,000 | 10,000
Asset Backed Securities |  |
Asset: |  |
Asset, fair value | 8,816 | 8,599
Asset Backed Securities | Level 2 |  |
Asset: |  |
Asset, fair value | 8,816 | 8,599
Certificate of Deposit |  |
Asset: |  |
Asset, fair value | 21,553 | 6,125
Certificate of Deposit | Level 2 |  |
Asset: |  |
Asset, fair value | 21,553 | 6,125
Non-US Government |  |
Asset: |  |
Asset, fair value | 12,900 | 5,240
Non-US Government | Level 2 |  |
Asset: |  |
Asset, fair value | $ 12,900 | $ 5,240

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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123874694&loc=d3e19207-110258 + Details
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