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Derivative Instruments (Total Fair Value Of Company's Derivative Assets And Liabilities By Instrument And Balance Sheet Location, Before Counterparty Netting) (Detail) (Not Designated as Hedging Instrument [Member], USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Derivative [Line Items] |  |  |  |
Derivative notional amount | $ 15,839 invest_DerivativeNotionalAmount |  | $ 31,316 invest_DerivativeNotionalAmount |
Derivative Liabilities [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Liabilities, Not designated, Fair Value | 537 mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeLiabilitiesMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1] | (1,347)mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeLiabilitiesMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1]
Derivative Assets [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Assets, Not designated, Fair Value | 91 mbi_DerivativeAssetsNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeAssetsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1] | 55 mbi_DerivativeAssetsNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeAssetsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1]
Interest Rate Swap [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative notional amount | 1,450 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 1,495 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
Interest Rate Swap [Member] | Derivative Liabilities [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Liabilities, Not designated, Fair Value | (248)mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1] | (165)mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1]
Interest Rate Swap [Member] | Other Assets [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Assets, Not designated, Fair Value | 83 mbi_DerivativeAssetsNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1] | 46 mbi_DerivativeAssetsNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1]
Credit Default Swap [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative notional amount | 9,481 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CreditDefaultSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 23,954 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CreditDefaultSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
Credit Default Swap [Member] | Derivative Liabilities [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Liabilities, Not designated, Fair Value | (244)mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CreditDefaultSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1] | (1,132)mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CreditDefaultSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1]
Insured Swaps [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative notional amount | 4,044 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_InsuredSwapsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 4,945 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_InsuredSwapsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
Insured Swaps [Member] | Derivative Liabilities [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Liabilities, Not designated, Fair Value | (2)mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = mbi_InsuredSwapsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1] | (5)mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = mbi_InsuredSwapsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1]
Interest Rate Swaps Embedded [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative notional amount | 437 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_InterestRateSwapsEmbeddedMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 496 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_InterestRateSwapsEmbeddedMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
Interest Rate Swaps Embedded [Member] | Medium Term Notes [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Assets, Not designated, Fair Value | 8 mbi_DerivativeAssetsNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_MediumTermNotesMember / us-gaap_DerivativeInstrumentRiskAxis = mbi_InterestRateSwapsEmbeddedMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1] | 9 mbi_DerivativeAssetsNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_MediumTermNotesMember / us-gaap_DerivativeInstrumentRiskAxis = mbi_InterestRateSwapsEmbeddedMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1]
Derivative Liabilities, Not designated, Fair Value | (19)mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_MediumTermNotesMember / us-gaap_DerivativeInstrumentRiskAxis = mbi_InterestRateSwapsEmbeddedMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1] | (19)mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_MediumTermNotesMember / us-gaap_DerivativeInstrumentRiskAxis = mbi_InterestRateSwapsEmbeddedMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1]
Currency Swaps Vie [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative notional amount | 91 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_CurrencySwapsVieMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 99 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_CurrencySwapsVieMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
Currency Swaps Vie [Member] | Derivative Liabilities V I E [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Liabilities, Not designated, Fair Value |  |  | (11)mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeLiabilitiesVIEMember / us-gaap_DerivativeInstrumentRiskAxis = mbi_CurrencySwapsVieMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1]
All Other [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative notional amount | 83 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_AllOtherMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 36 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_AllOtherMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
All Other [Member] | Derivative Liabilities [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative Liabilities, Not designated, Fair Value | (24)mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = mbi_AllOtherMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1] | (15)mbi_DerivativeLiabilitiesNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeLiabilitiesMember / us-gaap_DerivativeInstrumentRiskAxis = mbi_AllOtherMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | [1]
All Other V I E [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative notional amount | 241 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_AllOtherVIEMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | 280 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_AllOtherVIEMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
All Other Embedded [Member] |  |  |  |
Derivative [Line Items] |  |  |  |
Derivative notional amount | $ 12 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_AllOtherEmbeddedMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |  | $ 11 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_AllOtherEmbeddedMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember |
[1] | In accordance with the accounting guidance for derivative instruments and hedging activities, the balance sheet location of the Companys embedded derivative instruments is determined by the location of the related host contract

X
- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Fair value of derivative Asset Not Designated As Hedging Instrument, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position. + References No definition available. + Details
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- Definition Fair value of derivative liability Not Designated As Hedging Instrument, presented on a gross basis even when the derivative instrument is subject to master netting arrangements and qualifies for net presentation in the statement of financial position. + References No definition available. + Details
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