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Investments - Additional Information (Detail) - USD ($) | Mar. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Schedule of Investments [Line Items] |  |
Debt Securities, Available-for-sale, Allowance for Credit Loss | $ 0 |
Other investments | 363,000,000 | $ 478,000,000
Securities fair value | 7,000,000 | 6,000,000
Ambac Assurance [Member] |  |
Schedule of Investments [Line Items] |  |
Unsettled Amounts Secured Notes from LSNI | 31,000,000 | 55,000,000
Interest Rate Contract [Member] |  |
Schedule of Investments [Line Items] |  |
Other investments | 57,000,000 | $ 176,000,000
Commitments [Member] | Partnership Interest [Member] |  |
Schedule of Investments [Line Items] |  |
Other investments | $ 76,000,000 |

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