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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
Mar. 31, 2015
Dec. 31, 2014
Available For Sale Securities [Abstract]    
Amortized cost $ 6,444us-gaap_AvailableForSaleSecuritiesAmortizedCost $ 6,509us-gaap_AvailableForSaleSecuritiesAmortizedCost
Gross unrealized gains 246us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 209us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Gross unrealized losses (151)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax (147)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Total available-for-sale, fair value 6,539us-gaap_AvailableForSaleSecurities 6,571us-gaap_AvailableForSaleSecurities
Other-than-temporary impairments (104)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities [1] (90)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities [1]
Held To Maturity Securities [Abstract]    
Amortized cost 2,742us-gaap_HeldToMaturitySecurities 2,757us-gaap_HeldToMaturitySecurities
Gross unrealized gains 75us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain 77us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Gross unrealized losses (182)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss (202)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Total Held-To-Maturity, fair value 2,635us-gaap_HeldToMaturitySecuritiesFairValue 2,632us-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 2,742us-gaap_HeldToMaturitySecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
2,757us-gaap_HeldToMaturitySecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Gross unrealized gains 75us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
77us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Gross unrealized losses (182)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
(202)us-gaap_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Total Held-To-Maturity, fair value 2,635us-gaap_HeldToMaturitySecuritiesFairValue
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
2,632us-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 227us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
422us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
Gross unrealized gains 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
Gross unrealized losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
Total available-for-sale, fair value 227us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
422us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
Other-than-temporary impairments 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
[1] 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= mbi_MoneyMarketSecuritiesMember
[1]
Perpetual Debt And Equity Securities [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 12us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
12us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
Gross unrealized gains 1us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
1us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
Gross unrealized losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
Total available-for-sale, fair value 13us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= mbi_PerpetualDebtAndEquitySecuritiesMember
13us-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 6,205us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
6,075us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 245us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
208us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized losses (151)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
(147)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value 6,299us-gaap_AvailableForSaleSecurities
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
6,136us-gaap_AvailableForSaleSecurities
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Other-than-temporary impairments (104)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1] (90)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 2,742us-gaap_HeldToMaturitySecurities
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
 
Total Held-To-Maturity, fair value 2,635us-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 680us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
631us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 43us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
39us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized losses (1)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
(3)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value 722us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USTreasuryAndGovernmentMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
667us-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
1,644us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 112us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
94us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized losses (4)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
(8)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value 1,752us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_USStatesAndPoliticalSubdivisionsMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
1,730us-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 289us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
283us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 8us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
7us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized losses (1)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value 296us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_ForeignGovernmentDebtMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
290us-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 2,018us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
1,984us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 55us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
44us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized losses (103)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
(92)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value 1,970us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
1,936us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Other-than-temporary impairments (100)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
/ invest_InvestmentAxis
= us-gaap_CorporateDebtSecuritiesMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1] (86)mbi_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 1,076us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
1,116us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 20us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
17us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized losses (4)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
(7)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value 1,092us-gaap_AvailableForSaleSecurities
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
1,126us-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
= mbi_ResidentialMortgageBackedAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
[1]
Fixed Maturities [Member] | Residential Mortgage-Backed Non-Agency [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 50us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedNonAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
54us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedNonAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
Gross unrealized gains 4us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
= mbi_ResidentialMortgageBackedNonAgencyMember
/ us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis
= us-gaap_FixedMaturitiesMember
3us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ invest_InvestmentAxis
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Gross unrealized losses (4)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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(4)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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Total available-for-sale, fair value 50us-gaap_AvailableForSaleSecurities
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53us-gaap_AvailableForSaleSecurities
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Other-than-temporary impairments (4)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
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[1] (4)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
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[1]
Fixed Maturities [Member] | Commercial Mortgage-Backed [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 20us-gaap_AvailableForSaleSecuritiesAmortizedCost
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19us-gaap_AvailableForSaleSecuritiesAmortizedCost
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Gross unrealized gains 1us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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1us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Gross unrealized losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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Total available-for-sale, fair value 21us-gaap_AvailableForSaleSecurities
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20us-gaap_AvailableForSaleSecurities
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Other-than-temporary impairments 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
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[1] 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
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[1]
Fixed Maturities [Member] | Collateralized Debt Obligations [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 96us-gaap_AvailableForSaleSecuritiesAmortizedCost
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113us-gaap_AvailableForSaleSecuritiesAmortizedCost
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Gross unrealized gains 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Gross unrealized losses (22)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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(21)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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Total available-for-sale, fair value 74us-gaap_AvailableForSaleSecurities
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92us-gaap_AvailableForSaleSecurities
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Other-than-temporary impairments 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
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[1] 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
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[1]
Fixed Maturities [Member] | Other Asset-Backed [Member]    
Available For Sale Securities [Abstract]    
Amortized cost 332us-gaap_AvailableForSaleSecuritiesAmortizedCost
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231us-gaap_AvailableForSaleSecuritiesAmortizedCost
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Gross unrealized gains 2us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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3us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
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Gross unrealized losses (12)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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(12)us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
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Total available-for-sale, fair value 322us-gaap_AvailableForSaleSecurities
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222us-gaap_AvailableForSaleSecurities
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Other-than-temporary impairments $ 0mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities
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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.