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Fair Value Of Financial Instruments (Realized And Unrealized Gains And Losses Included In Earnings Pertaining To Level 3 Assets And Liabilities) (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Unrealized gains (losses) on insured derivatives | $ 37 us-gaap_UnrealizedGainLossOnDerivatives | $ 838 us-gaap_UnrealizedGainLossOnDerivatives
Net gains (losses) on financial instruments at fair value and foreign exchange | 30 mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange | (55)mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange
Fair Value Inputs Level 3 [Member] | Total Gains Losses Included In Earnings [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Unrealized gains (losses) on insured derivatives | 37 us-gaap_UnrealizedGainLossOnDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 838 us-gaap_UnrealizedGainLossOnDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Realized gains and other settlements on insured derivatives | (9)mbi_RealizedGainsLossesAndOtherSettlementsOnInsuredDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | (369)mbi_RealizedGainsLossesAndOtherSettlementsOnInsuredDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Net gains (losses) on financial instruments at fair value and foreign exchange | 18 mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | (13)mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Total revenues | 49 us-gaap_FairValueMeasuredOnRecurringBasisGainLossIncludedInEarnings / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 433 us-gaap_FairValueMeasuredOnRecurringBasisGainLossIncludedInEarnings / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Fair Value Inputs Level 3 [Member] | Change In Unrealized Gains Losses For Period Included In Earnings For Assets And Liabilities Still Held [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Unrealized gains (losses) on insured derivatives | 37 us-gaap_UnrealizedGainLossOnDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 30 us-gaap_UnrealizedGainLossOnDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Net gains (losses) on financial instruments at fair value and foreign exchange | 20 mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | (13)mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Total revenues | 61 us-gaap_FairValueMeasuredOnRecurringBasisGainLossIncludedInEarnings / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 11 us-gaap_FairValueMeasuredOnRecurringBasisGainLossIncludedInEarnings / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Fair Value Inputs Level 3 [Member] | Variable Interest Entity Primary Beneficiary [Member] | Total Gains Losses Included In Earnings [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Net gains (losses) on financial instruments at fair value and foreign exchange | 3 mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | (23)mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Fair Value Inputs Level 3 [Member] | Variable Interest Entity Primary Beneficiary [Member] | Change In Unrealized Gains Losses For Period Included In Earnings For Assets And Liabilities Still Held [Member] |  |
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |
Net gains (losses) on financial instruments at fair value and foreign exchange | $ 4 mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | $ (6)mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember

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- Definition Change in the fair value of and realized gains and losses on sale of financial instruments and foreign exchange included in earnings. + References No definition available. + Details
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- Definition Realized gains and losses and recognized premiums and fees related to insurance policies on derivatives included in earnings. + References No definition available. + Details
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- Definition Amount of realized and unrealized gain (loss) included in earnings, which have arisen from the use of significant unobservable inputs (level 3) to measure fair value of assets, liabilities, and financial instruments classified in shareholders' equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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