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Fair Value Of Financial Instruments (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives, value under Direct Price Model and Dual-Default Model	7.00%mbi_PercentageOfInsuredCreditDerivativesValueUnderDirectPriceModelAndDualDefaultModel
Net insured derivative liability	$ 244us-gaap_DerivativeAssetsLiabilitiesAtFairValueNet	$ 1,200us-gaap_DerivativeAssetsLiabilitiesAtFairValueNet
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability	92mbi_ImpactOfNonPerformanceRiskOnInsuredDerivativeLiability	394mbi_ImpactOfNonPerformanceRiskOnInsuredDerivativeLiability
Percentage of level 3 assets at fair value in total assets measured at fair value value	20.00%mbi_ApproximatePercentageOfLevel3AssetsAtFairValueInTotalAssetsMeasuredAtFairValue	22.00%mbi_ApproximatePercentageOfLevel3AssetsAtFairValueInTotalAssetsMeasuredAtFairValue
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value	35.00%mbi_ApproximatePercentageOfLevel3LiabilitiesAtFairValueInTotalLiabilitiesMeasuredAtFairValue	61.00%mbi_ApproximatePercentageOfLevel3LiabilitiesAtFairValueInTotalLiabilitiesMeasuredAtFairValue
Transfers into Level 3	240mbi_TransfersIntoLevelThree	403mbi_TransfersIntoLevelThree
Transfers out of Level 3	140mbi_TransfersOutOfLevelThree	198mbi_TransfersOutOfLevelThree
Transfers into Level 2	140mbi_TransfersIntoLevelTwo	198mbi_TransfersIntoLevelTwo
Transfers out of Level 2	240mbi_TransfersOutOfLevelTwo	403mbi_TransfersOutOfLevelTwo
Transfers into or out of Level 1	$ 0mbi_TransfersIntoAndOutOfLevelOne	$ 0mbi_TransfersIntoAndOutOfLevelOne
B E T Valuation Model [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives, fair value using the BET model	93.00%mbi_PercentageOfInsuredCreditDerivativesValueUnderBetModel / us-gaap_FairValueConcentrationOfRiskDisclosureItemsAxis = mbi_BETValuationModelMember

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- Definition

Approximate percentage of level 3 assets at fair value in total assets measured at fair value.

+ References

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Approximate percentage of level 3 liabilities at fair value in total liabilities measured at fair value.

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- Definition

Impact Of Non Performance Risk On Insured Derivative Liability

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- Definition

Percentage Of Insured Credit Derivatives Value Under Bet Model

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Percentage of insured credit derivatives value under direct price model and dual default model

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- Definition

Transfers of assets measured at fair value on a recurring basis into and out of level one during the period.

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Transfers of assets measured at fair value on a recurring basis into level two during the period.

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Transfers of assets measured at fair value on a recurring basis out of level three during the period.

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Transfers of assets measured at fair value on a recurring basis out of level two during the period.

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- Definition

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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