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Contracts Accounted for as Financial Guaranty Insurance - Loss and LAE Reported on the Statements of Operations (Details) - USD ($) $ in Millions | 3 Months Ended
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Mar. 31, 2026 | Mar. 31, 2025
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items] |  |
Loss and loss adjustment expenses (benefit) | $ 17 | $ 40
Public finance | Financial Guarantee Insurance And Other Product Line |  |
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items] |  |
Loss and loss adjustment expenses (benefit) | 19 | 42
Public finance | Financial Guarantee Insurance And Other Product Line | United States |  |
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items] |  |
Loss and loss adjustment expenses (benefit) | 12 | 36
Public finance | Financial Guarantee Insurance And Other Product Line | Non-U.S. public finance |  |
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items] |  |
Loss and loss adjustment expenses (benefit) | 7 | 6
RMBS | Financial Guarantee Insurance And Other Product Line | United States |  |
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items] |  |
Loss and loss adjustment expenses (benefit) | (1) | 0
Other structured finance | Financial Guarantee Insurance And Other Product Line |  |
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items] |  |
Loss and loss adjustment expenses (benefit) | (1) | (2)
Structured finance | Financial Guarantee Insurance And Other Product Line |  |
Liability for Claims and Claims Adjustment Expense Including Salvage and Subrogation Recoverable [Line Items] |  |
Loss and loss adjustment expenses (benefit) | $ (2) | $ (2)

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