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Business Developments And Risks And Uncertainties (Narrative) (Detail) $ in Millions			1 Months Ended	9 Months Ended
	Jul. 01, 2025 USD ($)	Jan. 01, 2025 USD ($)	Aug. 31, 2025 USD ($)	Sep. 30, 2025 USD ($) 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]
Custodial receipts			$ 374
Percentage of principal amount of current bond claims			47.00%
Outstanding insured bonds				$ 565
National Public Finance Guarantee Corporation [Member] | Puerto Rico Electric Power Authority [Member]
Business Acquisition [Line Items]
Claims payments	$ 92	$ 13

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