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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 | 6 Months Ended |
--- | --- | --- | ---
Jun. 30, 2024 | Jun. 30, 2023 | Jun. 30, 2024 | Jun. 30, 2023 | Dec. 31, 2023
Investment [Line Items] |  |  |  |  |
Investment expense | $ (1) | $ (1) | $ (3) | $ (3) |
Net investment income | 36 | 35 | 78 | 69 |
Standard & Poor's, B- Rating [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Investment Owned, at Fair Value | 403 |  | 403 |  |
Standard & Poor's, B- Rating [Member] | Mortgage-Backed Securities, Other |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Investment Owned, at Fair Value | 244 |  | 244 |  |
Standard & Poor's, B- Rating [Member] | Asset-backed Securities [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Investment Owned, at Fair Value | 151 |  | 151 |  |
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 | 2 | 2 | 9 | 6 |
Fixed Income Investments [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Gross Investment Income, Operating | 22 | 19 | 45 | 35 |
Short-Term [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Gross Investment Income, Operating | 5 | 5 | 10 | 10 |
Other Investments [Member] |  |  |  |  |
Investment [Line Items] |  |  |  |  |
Gross Investment Income, Operating | $ 8 | $ 9 | $ 21 | $ 22 |

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