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Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Total premiums earned	$ 416mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives	$ 494mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives	$ 661mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	299mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_US	391mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_US	530mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_US
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	36mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_GB	34mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_GB	36mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = country_GB
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	7mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_EuropeExcludingUnitedKingdomMember	11mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_EuropeExcludingUnitedKingdomMember	15mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_EuropeExcludingUnitedKingdomMember
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	9mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_InternationallyDiversifiedMember	9mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_InternationallyDiversifiedMember	16mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_InternationallyDiversifiedMember
Central And South America [Member]
Segment Reporting Information [Line Items]
Total premiums earned	49mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_CentralAndSouthAmericaMember	36mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_CentralAndSouthAmericaMember	48mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_CentralAndSouthAmericaMember
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	4mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember	4mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember	5mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = us-gaap_AsiaMember
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 12mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_OtherLocationsMember	$ 9mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_OtherLocationsMember	$ 11mbi_PremiumsEarnedNetFinancialGuaranteeInsuranceContractsAndInsuredDerivatives / us-gaap_StatementGeographicalAxis = mbi_OtherLocationsMember

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Premiums Earned Net Financial Guarantee Insurance Contracts And Insured Derivatives

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