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Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | $ 416 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives | $ 494 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives | $ 661 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives
United States [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | 299 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_US | 391 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_US | 530 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_US
United Kingdom [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | 36 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_GB | 34 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_GB | 36 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_GB
Europe Excluding United Kingdom [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | 7 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_EuropeExcludingUnitedKingdomMember | 11 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_EuropeExcludingUnitedKingdomMember | 15 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_EuropeExcludingUnitedKingdomMember
Internationally Diversified [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | 9 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_InternationallyDiversifiedMember | 9 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_InternationallyDiversifiedMember | 16 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_InternationallyDiversifiedMember
Central And South America [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | 49 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_CentralAndSouthAmericaMember | 36 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_CentralAndSouthAmericaMember | 48 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_CentralAndSouthAmericaMember
Asia [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | 4 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember | 4 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember | 5 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember
Other [Member] |  |  |
Segment Reporting Information [Line Items] |  |  |
Total premiums earned | $ 12 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_OtherLocationsMember | $ 9 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_OtherLocationsMember | $ 11 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_OtherLocationsMember

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