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Income Taxes (Income Taxes and Shareholders' Equity) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Current taxes:
Federal	$ 0us-gaap_CurrentFederalTaxExpenseBenefit	$ 0us-gaap_CurrentFederalTaxExpenseBenefit	$ 3us-gaap_CurrentFederalTaxExpenseBenefit
State	10us-gaap_CurrentStateAndLocalTaxExpenseBenefit	8us-gaap_CurrentStateAndLocalTaxExpenseBenefit	2us-gaap_CurrentStateAndLocalTaxExpenseBenefit
Foreign	12us-gaap_CurrentForeignTaxExpenseBenefit	1us-gaap_CurrentForeignTaxExpenseBenefit	(6)us-gaap_CurrentForeignTaxExpenseBenefit
Deferred taxes:
Federal	48us-gaap_DeferredFederalIncomeTaxExpenseBenefit	165us-gaap_DeferredFederalIncomeTaxExpenseBenefit	350us-gaap_DeferredFederalIncomeTaxExpenseBenefit
State			0us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit
Foreign	2us-gaap_DeferredForeignIncomeTaxExpenseBenefit	(8)us-gaap_DeferredForeignIncomeTaxExpenseBenefit	15us-gaap_DeferredForeignIncomeTaxExpenseBenefit
Provision (benefit) for income taxes	72us-gaap_IncomeTaxExpenseBenefit	166us-gaap_IncomeTaxExpenseBenefit	364us-gaap_IncomeTaxExpenseBenefit
Income tax effects allocated directly to equity [Abstract]
Change in unrealized gains and losses on AFS securities	89mbi_ChangeInUnrealizedGainLossOnInvestments	(81)mbi_ChangeInUnrealizedGainLossOnInvestments	86mbi_ChangeInUnrealizedGainLossOnInvestments
Change in AFS securities with OTTI	(29)us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossTaxPortionAttributableToParent	4us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossTaxPortionAttributableToParent	42us-gaap_OtherThanTemporaryImpairmentLossesInvestmentsPortionInOtherComprehensiveIncomeLossTaxPortionAttributableToParent
Change in foreign currency translation	(13)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentTax	1us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentTax	(2)us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTranslationAdjustmentTax
Share-based compensation	2us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost	4us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost	7us-gaap_IncomeTaxReconciliationNondeductibleExpenseShareBasedCompensationCost
Total income taxes charged (credited) to shareholders' equity	49us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquity	(72)us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquity	133us-gaap_IncomeTaxEffectsAllocatedDirectlyToEquity
Total effect of income taxes	$ 121us-gaap_IncomeTaxExpenseBenefitIntraperiodTaxAllocation	$ 94us-gaap_IncomeTaxExpenseBenefitIntraperiodTaxAllocation	$ 497us-gaap_IncomeTaxExpenseBenefitIntraperiodTaxAllocation

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