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Accumulated Other Comprehensive Income (Details Of The Reclassification From AOCI) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Net gains (losses) on financial instruments at fair value and foreign exchange	$ 78mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange	$ 69mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange	$ 55mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange
Income (loss) before income taxes	641us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	416us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest	1,598us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest
Provision (benefit) for income taxes	72us-gaap_IncomeTaxExpenseBenefit	166us-gaap_IncomeTaxExpenseBenefit	364us-gaap_IncomeTaxExpenseBenefit
Net income (loss)	569us-gaap_NetIncomeLoss	250us-gaap_NetIncomeLoss	1,234us-gaap_NetIncomeLoss
Amounts reclassified from AOCI [Member]
Net income (loss)	(17)us-gaap_NetIncomeLoss / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember	16us-gaap_NetIncomeLoss / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember	(70)us-gaap_NetIncomeLoss / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember
Unrealized gains (losses) on AFS, net [Member] | Amounts reclassified from AOCI [Member]
Net gains (losses) on financial instruments at fair value and foreign exchange	(17)mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember	23mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember	(4)mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Net investment income	(4)us-gaap_NetInvestmentIncome / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember	(5)us-gaap_NetInvestmentIncome / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember	(104)us-gaap_NetInvestmentIncome / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Income (loss) before income taxes	(21)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember	18us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember	(108)us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Provision (benefit) for income taxes	(8)us-gaap_IncomeTaxExpenseBenefit / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember	7us-gaap_IncomeTaxExpenseBenefit / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember	(38)us-gaap_IncomeTaxExpenseBenefit / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Net income (loss)	(13)us-gaap_NetIncomeLoss / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember	11us-gaap_NetIncomeLoss / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember	(70)us-gaap_NetIncomeLoss / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember
Foreign currency translation, net [Member] | Amounts reclassified from AOCI [Member]
Net gains (losses) on financial instruments at fair value and foreign exchange	$ (4)mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember	$ 5mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember	$ 0mbi_NetGainsLossesOnFinancialInstrumentsAtFairValueAndForeignExchange / us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis = us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember / us-gaap_StatementEquityComponentsAxis = us-gaap_AccumulatedTranslationAdjustmentMember

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