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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
Derivative [Line Items]
Derivative notional amount	$ 15,839invest_DerivativeNotionalAmount		$ 31,316invest_DerivativeNotionalAmount
Derivative Liabilities [Member]
Derivative [Line Items]
Derivative Liabilities, Not designated, Fair Value	537mbi_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	91mbi_DerivativeAssetsNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeAssetsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember	[1]	55mbi_DerivativeAssetsNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = mbi_DerivativeAssetsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember	[1]
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative notional amount	1,450invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember		1,495invest_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	83mbi_DerivativeAssetsNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_OtherAssetsMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember	[1]	46mbi_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	9,481invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CreditDefaultSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember		23,954invest_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	4,044invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_InsuredSwapsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember		4,945invest_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	437invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_InterestRateSwapsEmbeddedMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember		496invest_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	8mbi_DerivativeAssetsNotDesignatedAsHedgingInstrumentFairValue / us-gaap_BalanceSheetLocationAxis = us-gaap_MediumTermNotesMember / us-gaap_DerivativeInstrumentRiskAxis = mbi_InterestRateSwapsEmbeddedMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember	[1]	9mbi_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	91invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_CurrencySwapsVieMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember		99invest_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	83invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_AllOtherMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember		36invest_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	241invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_AllOtherVIEMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember		280invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_AllOtherVIEMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
All Other Embedded [Member]
Derivative [Line Items]
Derivative notional amount	$ 12invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = mbi_AllOtherEmbeddedMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember		$ 11invest_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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X

- 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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X

- 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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