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Investments (Average Credit Ratings For Securities Rated Below Investment Grade Without The Effect Of Financial Guarantees) (Detail) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015
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Asset-backed [Member] |
Schedule Of Investments [Line Items] |
Fair Value Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors | $ 56 mbi_FairValueOfSecuritiesRatedBelowInvestmentGradeWithoutGivingEffectToBenefitOfGuaranteesProvidedByFinancialGuarantors / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Percentage Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors | 57.00%mbi_PercentageOfSecuritiesRatedBelowInvestmentGradeWithoutGivingEffectToBenefitOfGuaranteesProvidedByFinancialGuarantors / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Mortgage-backed [Member] |
Schedule Of Investments [Line Items] |
Fair Value Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors | 7 mbi_FairValueOfSecuritiesRatedBelowInvestmentGradeWithoutGivingEffectToBenefitOfGuaranteesProvidedByFinancialGuarantors / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Percentage Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors | 100.00%mbi_PercentageOfSecuritiesRatedBelowInvestmentGradeWithoutGivingEffectToBenefitOfGuaranteesProvidedByFinancialGuarantors / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Corporate Obligations [Member] |
Schedule Of Investments [Line Items] |
Fair Value Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors | $ 0 mbi_FairValueOfSecuritiesRatedBelowInvestmentGradeWithoutGivingEffectToBenefitOfGuaranteesProvidedByFinancialGuarantors / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Percentage Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors | 0.00%mbi_PercentageOfSecuritiesRatedBelowInvestmentGradeWithoutGivingEffectToBenefitOfGuaranteesProvidedByFinancialGuarantors / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember

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- Definition Fair Value of Securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors. + References No definition available. + Details
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- Definition Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors. + References No definition available. + Details
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