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Investments - Summary of Sources of Collateral Received and Various Investment Agreement in which Collateral Pledged (Detail) - USD ($) $ in Millions | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
--- | --- | --- | ---
Investment [Line Items] |  |  |
Fair Value Of Securities Deposited With Governmental Authorities | $ 8 | $ 6 |
Market Value of Secured Note Collateral | 178 | 197 |
Restricted Cash | 13 | 55 | $ 19
Ambac Assurance [Member] |  |  |
Investment [Line Items] |  |  |
Restricted Cash | $ 9 | $ 55 |

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