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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
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 $ 56mbi_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 7mbi_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 $ 0mbi_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