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Investments (Amortized Cost And Fair Value Of Available-For-Sale and Held-To-Maturity Investment Portfolios) (Detail) (USD $)
In Millions, unless otherwise specified
Dec. 31, 2014
Dec. 31, 2013
Available For Sale Securities [Abstract]    
Amortized cost $ 6,509us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 6,824us-gaap_AvailableForSaleSecuritiesAmortizedCost
Gross unrealized gains 209mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments 97mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
Gross unrealized losses (147)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments (229)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Total available-for-sale, fair value 6,571us-gaap_AvailableForSaleSecurities 6,692us-gaap_AvailableForSaleSecurities
Other-than-temporary impairments (90)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities [1] (10)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities [1]
Held To Maturity Securities [Abstract]    
Amortized cost 2,757us-gaap_HeldToMaturitySecurities 2,801us-gaap_HeldToMaturitySecurities
Gross unrealized gains 77mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain 31mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Gross unrealized losses (202)mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss (181)mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Total Held-To-Maturity, fair value 2,632us-gaap_HeldToMaturitySecuritiesFairValue 2,651us-gaap_HeldToMaturitySecuritiesFairValue
Other-Than-Temporary Impairments 0mbi_HeldToMaturityCumulativeOtherThanTemporaryImpairmentOfSecurities [1] 0mbi_HeldToMaturityCumulativeOtherThanTemporaryImpairmentOfSecurities [1]
Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member]    
Held To Maturity Securities [Abstract]    
Amortized cost 2,757us-gaap_HeldToMaturitySecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
2,801us-gaap_HeldToMaturitySecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Gross unrealized gains 77mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
31mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Gross unrealized losses (202)mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
(181)mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Total Held-To-Maturity, fair value 2,632us-gaap_HeldToMaturitySecuritiesFairValue
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
2,651us-gaap_HeldToMaturitySecuritiesFairValue
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Other-Than-Temporary Impairments 0mbi_HeldToMaturityCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
[1] 0mbi_HeldToMaturityCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
[1]
Money Market Securities [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 422us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
781us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
Gross unrealized gains 0mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
0mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
Gross unrealized losses 0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
Total available-for-sale, fair value 422us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
781us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
Other-than-temporary impairments 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
[1] 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
[1]
Money Market Securities [Member] | Variable Interest Entity Primary Beneficiary [Member]    
Available For Sale Securities [Abstract]    
Amortized cost   136us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Gross unrealized gains   0mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Gross unrealized losses   0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Total available-for-sale, fair value   136us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Other-than-temporary impairments   0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
[1]
Perpetual Debt And Equity Securities [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 12us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
10us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
Gross unrealized gains 1mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
1mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
Gross unrealized losses 0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
Total available-for-sale, fair value 13us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
11us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
Other-than-temporary impairments 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
[1] 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
[1]
Fixed Maturities [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 6,075us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
5,897us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 208mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
96mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized losses (147)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
(229)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value 6,136us-gaap_AvailableForSaleSecurities
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
5,764us-gaap_AvailableForSaleSecurities
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Other-than-temporary impairments (90)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1] (10)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1]
Fixed Maturities [Member] | Variable Interest Entity Primary Beneficiary [Member]    
Held To Maturity Securities [Abstract]    
Amortized cost 2,757us-gaap_HeldToMaturitySecurities
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
 
Total Held-To-Maturity, fair value 2,632us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
 
Fixed Maturities [Member] | U S Treasury And Government [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 631us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
528us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 39mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
16mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized losses (3)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
(9)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value 667us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
535us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Other-than-temporary impairments 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1] 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1]
Fixed Maturities [Member] | US States And Political Subdivisions [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 1,644us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
1,831us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 94mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
22mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized losses (8)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
(73)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value 1,730us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
1,780us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Other-than-temporary impairments 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1] 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1]
Fixed Maturities [Member] | Foreign Governments [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 283us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
184us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 7mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
5mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized losses 0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value 290us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
189us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Other-than-temporary impairments 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1] 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1]
Fixed Maturities [Member] | Corporate Obligations [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 1,984us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
1,682us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 44mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
24mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized losses (92)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
(38)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value 1,936us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
1,668us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Other-than-temporary impairments (86)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1] 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1]
Fixed Maturities [Member] | Residential Mortgage-Backed Agency [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 1,116us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
1,167us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 17mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
10mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized losses (7)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
(31)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value 1,126us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
1,146us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Other-than-temporary impairments 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1] 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
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[1]
Fixed Maturities [Member] | Residential Mortgage-Backed Non-Agency [Member]    
Available For Sale Securities [Abstract]    
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[1] 4mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
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[1]
Fixed Maturities [Member] | Commercial Mortgage-Backed [Member]    
Available For Sale Securities [Abstract]    
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[1] 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
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[1]
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]    
Available For Sale Securities [Abstract]    
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[1] (14)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
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[1]
Fixed Maturities [Member] | Other Asset-Backed [Member]    
Available For Sale Securities [Abstract]    
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[1] $ 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
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[1]
[1]
Represents unrealized gains or losses on other than temporarily impaired securities recognized in AOCI, which includes the non-credit component of impairments, as well as all subsequent changes in fair value of such impaired securities reported in AOCI.