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Business Developments And Risks And Uncertainties (Narrative) (Detail) $ in Millions |  |  | 6 Months Ended
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Jul. 01, 2025 USD ($) | Jan. 01, 2025 USD ($) | Jun. 30, 2025 USD ($) Segments
Business Acquisition [Line Items] |  |  |
Number of operating segments | Segments |  |  | 3
MBIA Mexico [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Capital returned by subsidiary |  |  | $ 12
Puerto Rico Electric Power Authority [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Outstanding insured bonds |  |  | $ 657
National Public Finance Guarantee Corporation [Member] | Puerto Rico Electric Power Authority [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Claims payments |  | $ 13 |
Subsequent Event [Member] | National Public Finance Guarantee Corporation [Member] | Puerto Rico Electric Power Authority [Member] |  |  |
Business Acquisition [Line Items] |  |  |
Claims payments | $ 92 |  |

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