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Investments - Summary of Fair Value, Including Financial Guarantee, and Weighted-Average Underlying Rating, Excluding Financial Guarantee, of Insured Securities (Phantom) (Details) - USD ($) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2024 | Mar. 31, 2023 | Dec. 31, 2023
Investment [Line Items] |  |  |
Investment expense | $ (1) | $ (2) |
Net investment income | 42 | 34 |
Standard & Poor's, B- Rating [Member] |  |  |
Investment [Line Items] |  |  |
Investment Owned, at Fair Value | 406 |  |
Standard & Poor's, B- Rating [Member] | Mortgage-Backed Securities, Other |  |  |
Investment [Line Items] |  |  |
Investment Owned, at Fair Value | 244 |  |
Standard & Poor's, B- Rating [Member] | Asset-backed Securities [Member] |  |  |
Investment [Line Items] |  |  |
Investment Owned, at Fair Value | 154 |  |
Standard & Poor's, B Rating [Member] |  |  |
Investment [Line Items] |  |  |
Investment Owned, at Fair Value |  |  | $ 482
Standard & Poor's, B Rating [Member] | Mortgage-Backed Securities, Other |  |  |
Investment [Line Items] |  |  |
Investment Owned, at Fair Value |  |  | 240
Standard & Poor's, B Rating [Member] | Asset-backed Securities [Member] |  |  |
Investment [Line Items] |  |  |
Investment Owned, at Fair Value |  |  | $ 232
Gains (losses) on securities held as of reporting date [Member] |  |  |
Investment [Line Items] |  |  |
Other Investments Income | 7 | 5 |
Fixed Income Investments [Member] |  |  |
Investment [Line Items] |  |  |
Gross Investment Income, Operating | 23 | 17 |
Short-Term [Member] |  |  |
Investment [Line Items] |  |  |
Gross Investment Income, Operating | 5 | 6 |
Other Investments [Member] |  |  |
Investment [Line Items] |  |  |
Gross Investment Income, Operating | $ 13 | $ 13 |

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