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Derivative Instruments (Narrative) (Detail) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Derivative [Line Items]    
Maximum amount of future guarantee payments $ 9,700,000,000mbi_GuaranteeObligationsMaximumNetExposure  
Insured derivatives ceded under reinsurance agreements 13,000,000mbi_DerivativesCededUnderReinsuranceAgreements  
Cash collateral posted to derivative counterparties 12,000,000us-gaap_DerivativeCollateralRightToReclaimCash 160,000,000us-gaap_DerivativeCollateralRightToReclaimCash
Securities posted as collateral to derivative counterparties 198,000,000us-gaap_DerivativeCollateralRightToReclaimSecurities 42,000,000us-gaap_DerivativeCollateralRightToReclaimSecurities
Number of credit support annexes 1mbi_NumberOfCreditSupportAnnexes 1mbi_NumberOfCreditSupportAnnexes
Fair value of Credit Support Annex 2,000,000mbi_FairValueOfCreditSupportAnnexes 4,000,000mbi_FairValueOfCreditSupportAnnexes
Amount of counterparty netting 81,000,000mbi_DerivativeAssetCounterpartyNettingAmountFairValue 42,000,000mbi_DerivativeAssetCounterpartyNettingAmountFairValue
Derivative assets, after counterparty netting 10,000,000mbi_DerivativeFairValueOfDerivativeAssetNetOfCounterpartyNetting 13,000,000mbi_DerivativeFairValueOfDerivativeAssetNetOfCounterpartyNetting
Other assets and other assets of consolidated VIEs, after counterparty netting and embedded reclass 2,000,000mbi_OtherAssetsAndDerivativeAssetsVieAfterCounterpartyNettingAndEmbeddedReclass 4,000,000mbi_OtherAssetsAndDerivativeAssetsVieAfterCounterpartyNettingAndEmbeddedReclass
Counterparty netting 81,000,000mbi_DerivativeLiabilityCounterpartyNetting 42,000,000mbi_DerivativeLiabilityCounterpartyNetting
Derivative liabilities, after counterparty netting and posting of cash collateral 456,000,000mbi_DerivativeFairValueOfDerivativeLiabilityNetOfCounterpartyNetting 1,200,000,000mbi_DerivativeFairValueOfDerivativeLiabilityNetOfCounterpartyNetting
Derivative liabilities and derivative liabilities of consolidated VIEs, after counterparty netting and embedded reclass 437,000,000mbi_OtherLiabilitiesAndDerivativeLiabilitiesVieAfterCounterpartyNettingAndEmbeddedReclass  
Derivative Liabilities [Member]    
Derivative [Line Items]    
Cash collateral posted to derivative counterparties 0us-gaap_DerivativeCollateralRightToReclaimCash
/ us-gaap_BalanceSheetLocationAxis
= mbi_DerivativeLiabilitiesMember		 123,000,000us-gaap_DerivativeCollateralRightToReclaimCash
/ us-gaap_BalanceSheetLocationAxis
= mbi_DerivativeLiabilitiesMember
Other Liabilities [Member]    
Derivative [Line Items]    
Cash collateral posted to derivative counterparties 12,000,000us-gaap_DerivativeCollateralRightToReclaimCash
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember		 16,000,000us-gaap_DerivativeCollateralRightToReclaimCash
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherLiabilitiesMember
Other Assets [Member]    
Derivative [Line Items]    
Cash collateral posted to derivative counterparties   21,000,000us-gaap_DerivativeCollateralRightToReclaimCash
/ us-gaap_BalanceSheetLocationAxis
= us-gaap_OtherAssetsMember
Medium Term Notes And Other Investments [Member] | Embedded [Member]    
Derivative [Line Items]    
Embedded derivative assets 8,000,000us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_BalanceSheetLocationAxis
= mbi_MediumTermNotesAndOtherInvestmentsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= mbi_EmbeddedMember		 9,000,000us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_BalanceSheetLocationAxis
= mbi_MediumTermNotesAndOtherInvestmentsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= mbi_EmbeddedMember
Total derivative liabilities $ 19,000,000us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_BalanceSheetLocationAxis
= mbi_MediumTermNotesAndOtherInvestmentsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= mbi_EmbeddedMember		 $ 19,000,000us-gaap_DerivativeFairValueOfDerivativeLiability
/ us-gaap_BalanceSheetLocationAxis
= mbi_MediumTermNotesAndOtherInvestmentsMember
/ us-gaap_DerivativeInstrumentRiskAxis
= mbi_EmbeddedMember