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Investments - Internally Managed Investment Portfolio (Details) - USD ($) $ in Millions | Jun. 30, 2023 | Dec. 31, 2022
--- | --- | ---
Debt and Equity Securities, FV-NI [Line Items] |  |
Estimated fair value | $ 6,488 | $ 7,119
Fixed-maturity securities, trading | 340 | 303
Short-term investments, at fair value | 1,650 | 810
Other invested assets | 146 | 133
Total investments | 8,624 | 8,365
Externally managed |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Estimated fair value | 5,551 | 5,824
Externally managed | Assured IM |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Estimated fair value | 292 | 305
Loss Mitigation Securities |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Estimated fair value | 509 | 548
Puerto Rico, New Recovery Bonds |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Estimated fair value | 36 | 358
Other |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Estimated fair value | 392 | 389
Other | Assured IM |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Estimated fair value | $ 221 | $ 232
BIG |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed-maturity investments, non-investment grade, percent | 7.50% | 7.40%
Not Internally Rated |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Fixed-maturity investments, non-investment grade, percent | 1.70% | 5.90%
Equity method investments |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other invested assets | $ 130 | $ 123
Other |  |
Debt and Equity Securities, FV-NI [Line Items] |  |
Other invested assets | $ 16 | $ 10

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