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Fair Value Of Financial Instruments (Company's Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | $ 10,368 us-gaap_AssetsFairValueDisclosureRecurring |  | $ 10,613 us-gaap_AssetsFairValueDisclosureRecurring |
Fair value financial liabilities measured on recurring basis | 3,369 us-gaap_LiabilitiesFairValueDisclosureRecurring |  | 3,463 us-gaap_LiabilitiesFairValueDisclosureRecurring |
Variable Interest Entity Primary Beneficiary [Member] | Residential Loans Receivable [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1,372 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_ResidentialPrimeFinancingReceivableMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 1,431 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_ResidentialPrimeFinancingReceivableMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Variable Interest Entity Primary Beneficiary [Member] | Other Loans Receivable [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 108 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  |
Variable Interest Entity Primary Beneficiary [Member] | Loan Repurchase Commitments [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 385 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 379 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 63 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 65 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 192 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 197 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 737 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 86 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Collateralized Debt Obligations [Member] | Asset-backed [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 6 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 12 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Other Asset Backed [Member] | Asset-backed [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 63 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 61 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Money Market Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 240 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember |  | 428 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember |
Medium Term Notes [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 180 us-gaap_LiabilitiesFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_MediumTermNotesMember |  | 197 us-gaap_LiabilitiesFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_MediumTermNotesMember |
Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 2,653 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 2,047 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Perpetual Debt And Equity Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 51 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  | 53 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |
Cash And Cash Equivalents [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 596 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CashAndCashEquivalentsMember |  | 729 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CashAndCashEquivalentsMember |
Fixed Maturities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 6,518 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 6,345 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fixed Maturities [Member] | U S Treasury And Government [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 744 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 691 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fixed Maturities [Member] | State and municipal bonds [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1,756 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 1,732 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fixed Maturities [Member] | Foreign Government Debt [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 296 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 290 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fixed Maturities [Member] | Corporate Obligations [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 2,098 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 2,058 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1,129 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 1,162 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 52 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 56 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 23 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 22 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fixed Maturities [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 75 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 93 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fixed Maturities [Member] | Other Asset Backed [Member] | Asset-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 345 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 241 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Derivative Assets [Member] | Interest Rate Derivatives [Member] | Non Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByLiabilityClassAxis = mbi_NonInsuredDerivativesMember / invest_InvestmentAxis = mbi_InterestRateDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeAssetsMember |  | 2 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByLiabilityClassAxis = mbi_NonInsuredDerivativesMember / invest_InvestmentAxis = mbi_InterestRateDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeAssetsMember |
Derivative Assets [Member] | Currency Derivatives [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 5 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_CurrencyDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeAssetsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  |
Assets Held For Sale [Member] | Loans Receivable At Fair Value [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  |  | 711 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_LoansreceivableatfairvalueMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_AssetsHeldForSaleMember |
Assets Held For Sale [Member] | Equity Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  |  | 6 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_EquitySecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_AssetsHeldForSaleMember |
Assets Held For Sale [Member] | Cash And Cash Equivalents [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  |  | 55 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_AssetsHeldForSaleMember |
Assets Of Consolidated V I Es [Member] | Cash And Cash Equivalents [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 31 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_AssetsOfConsolidatedVIEsMember |  | 53 us-gaap_AssetsFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_AssetsOfConsolidatedVIEsMember |
Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 210 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByLiabilityClassAxis = mbi_InsuredDerivativesMember / invest_InvestmentAxis = mbi_CreditDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeLiabilitiesMember |  | 246 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByLiabilityClassAxis = mbi_InsuredDerivativesMember / invest_InvestmentAxis = mbi_CreditDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeLiabilitiesMember |
Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Non Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 160 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByLiabilityClassAxis = mbi_NonInsuredDerivativesMember / invest_InvestmentAxis = mbi_InterestRateDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeLiabilitiesMember |  | 167 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByLiabilityClassAxis = mbi_NonInsuredDerivativesMember / invest_InvestmentAxis = mbi_InterestRateDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeLiabilitiesMember |
Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 76 us-gaap_LiabilitiesFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_InterestRateDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeLiabilitiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  |
Derivative Liabilities [Member] | Other Derivatives [Member] | Non Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 21 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByLiabilityClassAxis = mbi_NonInsuredDerivativesMember / invest_InvestmentAxis = mbi_OtherDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeLiabilitiesMember |  | 24 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByLiabilityClassAxis = mbi_NonInsuredDerivativesMember / invest_InvestmentAxis = mbi_OtherDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeLiabilitiesMember |
Other Liabilities [Member] | Warrant [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 33 us-gaap_LiabilitiesFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_WarrantMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_OtherLiabilitiesMember |  | 28 us-gaap_LiabilitiesFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_WarrantMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_OtherLiabilitiesMember |
Other Liabilities [Member] | Investments Payable [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 36 us-gaap_LiabilitiesFairValueDisclosureRecurring / invest_InvestmentAxis = us-gaap_DebtSecuritiesPayableMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_OtherLiabilitiesMember |  |  |
Liabilities Held For Sale [Member] | Variable Interest Entity Notes [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis |  |  | 431 us-gaap_LiabilitiesFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_LiabilitiesheldforsaleMember |
Liabilities Held For Sale [Member] | Payable For Loans Purchased [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis |  |  | 323 us-gaap_LiabilitiesFairValueDisclosureRecurring / invest_InvestmentAxis = mbi_PayableforloanspurchasedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_LiabilitiesheldforsaleMember |
Fair Value Inputs Level 1 [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1,725 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 2,087 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Fair value financial liabilities measured on recurring basis | 36 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |  | 0 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member |
Fair Value Inputs Level 1 [Member] | Money Market Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 240 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember |  | 428 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember |
Fair Value Inputs Level 1 [Member] | Perpetual Debt And Equity Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 24 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  | 22 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |
Fair Value Inputs Level 1 [Member] | Cash And Cash Equivalents [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 596 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = us-gaap_CashAndCashEquivalentsMember |  | 729 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = us-gaap_CashAndCashEquivalentsMember |
Fair Value Inputs Level 1 [Member] | Fixed Maturities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 834 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 794 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 1 [Member] | Fixed Maturities [Member] | U S Treasury And Government [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 624 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 573 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 1 [Member] | Fixed Maturities [Member] | Foreign Government Debt [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 210 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 221 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 1 [Member] | Assets Held For Sale [Member] | Equity Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  |  | 6 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = us-gaap_EquitySecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_AssetsHeldForSaleMember |
Fair Value Inputs Level 1 [Member] | Assets Held For Sale [Member] | Cash And Cash Equivalents [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  |  | 55 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_AssetsHeldForSaleMember |
Fair Value Inputs Level 1 [Member] | Assets Of Consolidated V I Es [Member] | Cash And Cash Equivalents [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 31 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_AssetsOfConsolidatedVIEsMember |  | 53 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = us-gaap_CashAndCashEquivalentsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_AssetsOfConsolidatedVIEsMember |
Fair Value Inputs Level 1 [Member] | Other Liabilities [Member] | Investments Payable [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 36 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel1Member / invest_InvestmentAxis = us-gaap_DebtSecuritiesPayableMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_OtherLiabilitiesMember |  |  |
Fair Value Inputs Level 2 [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 6,522 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 6,510 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Fair value financial liabilities measured on recurring basis | 1,647 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |  | 2,344 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member |
Fair Value Inputs Level 2 [Member] | Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 10 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 10 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Fair Value Inputs Level 2 [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 189 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 194 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Fair Value Inputs Level 2 [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 737 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 86 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Fair Value Inputs Level 2 [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 2 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 7 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Fair Value Inputs Level 2 [Member] | Other Asset Backed [Member] | Asset-backed [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 42 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 35 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Fair Value Inputs Level 2 [Member] | Variable Interest Entity Notes [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 1,266 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 1,312 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Fair Value Inputs Level 2 [Member] | Perpetual Debt And Equity Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 27 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  | 31 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 5,513 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 5,353 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | U S Treasury And Government [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 120 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 118 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | State and municipal bonds [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1,750 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 1,724 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Foreign Government Debt [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 80 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 63 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Corporate Obligations [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 2,088 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 2,048 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1,129 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 1,162 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 52 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 56 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] | Mortgage-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 22 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 20 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Collateralized Debt Obligations [Member] | Asset-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 1 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 6 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 2 [Member] | Fixed Maturities [Member] | Other Asset Backed [Member] | Asset-backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 271 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 156 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Fair Value Inputs Level 2 [Member] | Derivative Assets [Member] | Interest Rate Derivatives [Member] | Non Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis | 2 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByLiabilityClassAxis = mbi_NonInsuredDerivativesMember / invest_InvestmentAxis = mbi_InterestRateDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeAssetsMember |  | 83 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByLiabilityClassAxis = mbi_NonInsuredDerivativesMember / invest_InvestmentAxis = mbi_InterestRateDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeAssetsMember |
Fair Value Inputs Level 2 [Member] | Assets Held For Sale [Member] | Loans Receivable At Fair Value [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial assets measured on recurring basis |  |  | 711 us-gaap_AssetsFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / invest_InvestmentAxis = mbi_LoansreceivableatfairvalueMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_AssetsHeldForSaleMember |
Fair Value Inputs Level 2 [Member] | Derivative Liabilities [Member] | Credit Derivatives [Member] | Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 3 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByLiabilityClassAxis = mbi_InsuredDerivativesMember / invest_InvestmentAxis = mbi_CreditDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeLiabilitiesMember |  | 2 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByLiabilityClassAxis = mbi_InsuredDerivativesMember / invest_InvestmentAxis = mbi_CreditDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeLiabilitiesMember |
Fair Value Inputs Level 2 [Member] | Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Non Insured Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Fair value financial liabilities measured on recurring basis | 269 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByLiabilityClassAxis = mbi_NonInsuredDerivativesMember / invest_InvestmentAxis = mbi_InterestRateDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeLiabilitiesMember |  | 248 us-gaap_LiabilitiesFairValueDisclosureRecurring / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel2Member / us-gaap_FairValueByLiabilityClassAxis = mbi_NonInsuredDerivativesMember / invest_InvestmentAxis = mbi_InterestRateDerivativesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_DerivativeLiabilitiesMember |
Fair Value Inputs Level 2 [Member] | Derivative Liabilities [Member] | Interest Rate Derivatives [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] | State and municipal bonds [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Fair Value Inputs Level 3 [Member] | Fixed Maturities [Member] | Foreign Government Debt [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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Accounts Notes And Loans Receivable [Line Items] |  |  |  |
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[1] | Unobservable inputs are either not developed by the Company or do not significantly impact the overall fair values of the aggregate financial assets and liabilities.

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