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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 | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items] |  |  |
Unrealized gains (losses) on insured derivatives | $ 903 us-gaap_UnrealizedGainLossOnDerivatives | $ 1,777 us-gaap_UnrealizedGainLossOnDerivatives | $ 1,870 us-gaap_UnrealizedGainLossOnDerivatives
Net gains (losses) on financial instruments at fair value and foreign exchange | 78 mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange | 69 mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange | 55 mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange
Net investment losses related to other-than-temporary impairments | (15)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet |  | (105)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet
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 | 903 us-gaap_UnrealizedGainLossOnDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 1,778 us-gaap_UnrealizedGainLossOnDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 1,869 us-gaap_UnrealizedGainLossOnDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Realized gains and other settlements on insured derivatives | (445)mbi_RealizedGainsLossesAndOtherSettlementsOnInsuredDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | (1,548)mbi_RealizedGainsLossesAndOtherSettlementsOnInsuredDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | (407)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 | (37)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
Net investment losses related to other-than-temporary impairments |  |  | (68)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionRecognizedInEarningsNet / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Total revenues | 444 us-gaap_FairValueMeasuredOnRecurringBasisGainLossIncludedInEarnings / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 353 us-gaap_FairValueMeasuredOnRecurringBasisGainLossIncludedInEarnings / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 1,031 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 | 20 us-gaap_UnrealizedGainLossOnDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | (87)us-gaap_UnrealizedGainLossOnDerivatives / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 927 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 | 12 mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | (12)mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | (6)mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member
Total revenues | 52 us-gaap_FairValueMeasuredOnRecurringBasisGainLossIncludedInEarnings / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 161 us-gaap_FairValueMeasuredOnRecurringBasisGainLossIncludedInEarnings / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member | 650 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 | (32)mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 160 mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_TotalGainsLossesIncludedInEarningsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | (376)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 | $ 20 mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | $ 260 mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / mbi_FairValueByConsolidatedStatementsOfOperationsAxis = mbi_ChangeInUnrealizedGainsLossesForPeriodIncludedInEarningsForAssetsAndLiabilitiesStillHeldMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | $ (271)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=25499696&loc=d3e19207-110258 + Details
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- Definition The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8B -URI http://asc.fasb.org/extlink&oid=27724398&loc=SL6284393-111563 + 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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