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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 | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Segment Reporting Information [Line Items] |  |
Total premiums earned | $ 105 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives | $ 93 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives
United States [Member] |  |
Segment Reporting Information [Line Items] |  |
Total premiums earned | 87 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_US | 67 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_US
United Kingdom [Member] |  |
Segment Reporting Information [Line Items] |  |
Total premiums earned | 7 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_GB | 9 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_GB
Europe Excluding United Kingdom [Member] |  |
Segment Reporting Information [Line Items] |  |
Total premiums earned | 2 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_EuropeExcludingUnitedKingdomMember | 3 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_EuropeExcludingUnitedKingdomMember
Internationally Diversified [Member] |  |
Segment Reporting Information [Line Items] |  |
Total premiums earned | 1 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_InternationallyDiversifiedMember | 2 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_InternationallyDiversifiedMember
Central And South America [Member] |  |
Segment Reporting Information [Line Items] |  |
Total premiums earned | 6 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_CentralAndSouthAmericaMember | 9 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_CentralAndSouthAmericaMember
Asia [Member] |  |
Segment Reporting Information [Line Items] |  |
Total premiums earned | 1 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember | 1 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember
Other [Member] |  |
Segment Reporting Information [Line Items] |  |
Total premiums earned | $ 1 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_OtherLocationsMember | $ 2 mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_OtherLocationsMember

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- Definition Premiums Earned Net Financial Guarantee Insurance Contracts And Insured Derivatives + References No definition available. + Details
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Period Type: |

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