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Investments - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions | Sep. 30, 2023 | Aug. 31, 2023 | Dec. 31, 2022
--- | --- | --- | ---
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Estimated fair value | $ 6,267 |  | $ 7,119
Fixed-maturity securities, trading | 350 |  | 303
Short-term investments, at fair value | 1,426 |  | 810
Other invested assets | 765 |  | 133
Total investments | 8,808 |  | 8,365
Externally managed |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Estimated fair value | 5,316 |  | 5,824
Externally managed | Internal |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Estimated fair value | 294 |  | 305
Loss Mitigation Securities |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Estimated fair value | 495 |  | 548
Puerto Rico, New Recovery Bonds |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Estimated fair value | 78 |  | 358
Puerto Rico, New Recovery Bonds | AGC And AGM |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Recovery bonds transferred |  | $ 68 |
Other |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Estimated fair value | 378 |  | 389
Other | Assured IM |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Estimated fair value | $ 214 |  | $ 232
Contingent Value Instruments | AGC And AGM |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Recovery bonds transferred |  | $ 5 |
BIG |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Fixed-maturity investments, non-investment grade, percent | 7.60% |  | 7.40%
Not Internally Rated |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Fixed-maturity investments, non-investment grade, percent | 2.40% |  | 5.90%
Sound Point Capital Management, LP [Member] |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Other invested assets | $ 419 |  | $ 0
Equity method investments |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Other invested assets | 331 |  | 123
Other |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Other invested assets | $ 15 |  | $ 10

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