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Fair Value Of Financial Instruments (Quantitative Information Regarding The Significant Unobservable Inputs For Certain Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Other Derivative Liabilities [Member] | Discounted cash flow [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair value, liabilities | $ 21 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_ValuationTechniqueAxis = mbi_DiscountedCashFlowMember |  | $ 24 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_ValuationTechniqueAxis = mbi_DiscountedCashFlowMember
Discounted Cash Flows [Member] | Other Derivative Liabilities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range dollars | 0 mbi_FairValueMeasurementRangeDollars / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_DiscountedCashFlowsMember |  | 0 mbi_FairValueMeasurementRangeDollars / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_DiscountedCashFlowsMember
Discounted Cash Flows [Member] | Other Derivative Liabilities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range dollars | 83 mbi_FairValueMeasurementRangeDollars / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_DiscountedCashFlowsMember |  | 83 mbi_FairValueMeasurementRangeDollars / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_DiscountedCashFlowsMember
Discounted Cash Flows [Member] | Other Derivative Liabilities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range dollars | 42 mbi_FairValueMeasurementRangeDollars / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_DiscountedCashFlowsMember | [1] | 42 mbi_FairValueMeasurementRangeDollars / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_DiscountedCashFlowsMember
Credit Derivatives [Member] | Commercial Mortgage Backed Securities [Member] | BET Model [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair value, liabilities | 191 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ValuationTechniqueAxis = mbi_BETValuationModelMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 224 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ValuationTechniqueAxis = mbi_BETValuationModelMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Multi Sector C D O [Member] | Direct Price Model [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair value, liabilities | 6 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / invest_InvestmentAxis = mbi_MultiSectorCDOMember / us-gaap_ValuationTechniqueAxis = mbi_DirectPriceModelMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 9 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / invest_InvestmentAxis = mbi_MultiSectorCDOMember / us-gaap_ValuationTechniqueAxis = mbi_DirectPriceModelMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Other Derivative Liabilities [Member] | BET And Dual Default Valuation Models [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair value, liabilities | 10 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_ValuationTechniqueAxis = mbi_BetanddualdefaultvaluationmodelsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 11 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_ValuationTechniqueAxis = mbi_BetanddualdefaultvaluationmodelsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Recovery Rates [Member] | Commercial Mortgage Backed Securities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 25.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_RecoveryRatesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 25.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_RecoveryRatesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Recovery Rates [Member] | Commercial Mortgage Backed Securities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 90.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_RecoveryRatesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 90.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_RecoveryRatesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Recovery Rates [Member] | Commercial Mortgage Backed Securities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 61.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_RecoveryRatesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 59.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_RecoveryRatesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Recovery Rates [Member] | Other Derivative Liabilities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 42.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_RecoveryRatesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 42.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_RecoveryRatesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Recovery Rates [Member] | Other Derivative Liabilities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 45.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_RecoveryRatesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 45.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_RecoveryRatesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Recovery Rates [Member] | Other Derivative Liabilities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 45.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_RecoveryRatesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 45.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_RecoveryRatesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 19.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 12.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 32.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 29.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Nonperformance Risk [Member] | Commercial Mortgage Backed Securities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 28.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 25.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Nonperformance Risk [Member] | Multi Sector C D O [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 50.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_MultiSectorCDOMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 53.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_MultiSectorCDOMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Nonperformance Risk [Member] | Multi Sector C D O [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 50.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_MultiSectorCDOMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 53.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_MultiSectorCDOMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Nonperformance Risk [Member] | Multi Sector C D O [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 50.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_MultiSectorCDOMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 53.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_MultiSectorCDOMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Nonperformance Risk [Member] | Other Derivative Liabilities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 50.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 41.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Nonperformance Risk [Member] | Other Derivative Liabilities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 50.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 51.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Nonperformance Risk [Member] | Other Derivative Liabilities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 50.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 50.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_NonperformanceRiskMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Weighted Average Life [Member] | Commercial Mortgage Backed Securities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Weighted Average (in years) | 1 mbi_FairValueDisclosureWeightedAverageInYears / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_WeightedAverageLifeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 1.3 mbi_FairValueDisclosureWeightedAverageInYears / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_WeightedAverageLifeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Weighted Average Life [Member] | Commercial Mortgage Backed Securities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Weighted Average (in years) | 2 mbi_FairValueDisclosureWeightedAverageInYears / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_WeightedAverageLifeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 3.2 mbi_FairValueDisclosureWeightedAverageInYears / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_WeightedAverageLifeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Weighted Average Life [Member] | Commercial Mortgage Backed Securities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Weighted Average (in years) | 1.6 mbi_FairValueDisclosureWeightedAverageInYears / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_WeightedAverageLifeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 2.3 mbi_FairValueDisclosureWeightedAverageInYears / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_WeightedAverageLifeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Weighted Average Life [Member] | Other Derivative Liabilities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Weighted Average (in years) | 0.5 mbi_FairValueDisclosureWeightedAverageInYears / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_WeightedAverageLifeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 0.2 mbi_FairValueDisclosureWeightedAverageInYears / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_WeightedAverageLifeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Weighted Average Life [Member] | Other Derivative Liabilities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Weighted Average (in years) | 7.8 mbi_FairValueDisclosureWeightedAverageInYears / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_WeightedAverageLifeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 7.9 mbi_FairValueDisclosureWeightedAverageInYears / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_WeightedAverageLifeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Weighted Average Life [Member] | Other Derivative Liabilities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Weighted Average (in years) | 1 mbi_FairValueDisclosureWeightedAverageInYears / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_WeightedAverageLifeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 1.1 mbi_FairValueDisclosureWeightedAverageInYears / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_WeightedAverageLifeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Commercial Mortgage Backed Securities Spreads [Member] | Commercial Mortgage Backed Securities [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 0.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_CommercialMortgageBackedSecuritiesSpreadsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 0.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_CommercialMortgageBackedSecuritiesSpreadsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Commercial Mortgage Backed Securities Spreads [Member] | Commercial Mortgage Backed Securities [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 57.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_CommercialMortgageBackedSecuritiesSpreadsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 41.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_CommercialMortgageBackedSecuritiesSpreadsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Credit Derivatives [Member] | Commercial Mortgage Backed Securities Spreads [Member] | Commercial Mortgage Backed Securities [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 22.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_CommercialMortgageBackedSecuritiesSpreadsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember |  | 19.00%mbi_FairValueMeasurementRangePercentage / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_CommercialMortgageBackedSecuritiesSpreadsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_CreditDerivativesMember
Loans Receivable [Member] | Variable Interest Entity Primary Beneficiary [Member] | Quoted market prices adjusted for financial guarantees provided to VIE obligations [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair Value, assets | 1,480 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_ValuationTechniqueAxis = mbi_QuotedMarketPricesAdjustedForFinancialGuaranteesProvidedToVieObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 1,431 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_ValuationTechniqueAxis = mbi_QuotedMarketPricesAdjustedForFinancialGuaranteesProvidedToVieObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Loans Receivable [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 0.00%mbi_FairValueMeasurementRangePercentage / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_ImpactOfFinancialGuaranteeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 0.00%mbi_FairValueMeasurementRangePercentage / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_ImpactOfFinancialGuaranteeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Loans Receivable [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 6.00%mbi_FairValueMeasurementRangePercentage / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_ImpactOfFinancialGuaranteeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 10.00%mbi_FairValueMeasurementRangePercentage / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_ImpactOfFinancialGuaranteeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Loans Receivable [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 2.00%mbi_FairValueMeasurementRangePercentage / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_ImpactOfFinancialGuaranteeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 2.00%mbi_FairValueMeasurementRangePercentage / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_ImpactOfFinancialGuaranteeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Loan Repurchase Commitments [Member] | Variable Interest Entity Primary Beneficiary [Member] | Discounted cash flow [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair Value, assets | 385 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_ValuationTechniqueAxis = mbi_DiscountedCashFlowMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 379 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_ValuationTechniqueAxis = mbi_DiscountedCashFlowMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Quoted market prices of VIE assets adjusted for financial guarantees provided [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Fair value, liabilities | $ 1,387 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_VariableInterestEntityNotesMember / us-gaap_ValuationTechniqueAxis = mbi_QuotedMarketPricesOfVieAssetsAdjustedForFinancialGuaranteesProvidedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | $ 735 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_VariableInterestEntityNotesMember / us-gaap_ValuationTechniqueAxis = mbi_QuotedMarketPricesOfVieAssetsAdjustedForFinancialGuaranteesProvidedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Minimum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 0.00%mbi_FairValueMeasurementRangePercentage / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_VariableInterestEntityNotesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_ImpactOfFinancialGuaranteeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 0.00%mbi_FairValueMeasurementRangePercentage / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_VariableInterestEntityNotesMember / us-gaap_RangeAxis = us-gaap_MinimumMember / mbi_UnobservableInputAxis = mbi_ImpactOfFinancialGuaranteeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Maximum [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 39.00%mbi_FairValueMeasurementRangePercentage / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_VariableInterestEntityNotesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_ImpactOfFinancialGuaranteeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 35.00%mbi_FairValueMeasurementRangePercentage / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_VariableInterestEntityNotesMember / us-gaap_RangeAxis = us-gaap_MaximumMember / mbi_UnobservableInputAxis = mbi_ImpactOfFinancialGuaranteeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] | Impact Of Financial Guarantee [Member] | Weighted Average [Member] |  |  |
Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation [Line Items] |  |  |
Range percentage | 9.00%mbi_FairValueMeasurementRangePercentage / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_VariableInterestEntityNotesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_ImpactOfFinancialGuaranteeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 16.00%mbi_FairValueMeasurementRangePercentage / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_VariableInterestEntityNotesMember / us-gaap_RangeAxis = us-gaap_WeightedAverageMember / mbi_UnobservableInputAxis = mbi_ImpactOfFinancialGuaranteeMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
[1] | Midpoint of cash flows are used for the weighted average.

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