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Investments (Gross Unrealized Losses Related To Available-For-Sale And Held-To-Maturity Investments) (Detail) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | $ 818 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue | $ 851 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Less than 12 months, unrealized losses | (4)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss | (2)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
12 months or longer, fair value | 542 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue | 1,220 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
12 months or longer, unrealized losses | (147)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss | (145)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Total available-for-sale, fair value | 1,360 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue | 2,071 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Total available-for-sale, unrealized losses | (151)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss | (147)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Held To Maturity Securities [Abstract] |  |
Less than 12 months, fair value | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Less than 12 months, unrealized losses | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
12 months or longer, fair value | 393 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue | 373 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
12 months or longer, unrealized losses | (182)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss | (202)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Total held-to-maturity, fair value | 393 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue | 373 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Total held-to-maturity, unrealized losses | (182)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss | (202)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |
Held To Maturity Securities [Abstract] |  |
Less than 12 months, fair value | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Less than 12 months, unrealized losses | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 0 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
12 months or longer, fair value | 393 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 373 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
12 months or longer, unrealized losses | (182)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | (202)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Total held-to-maturity, fair value | 393 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 373 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Total held-to-maturity, unrealized losses | (182)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | (202)us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Perpetual Debt And Equity Securities [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 4 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember | 6 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember
Less than 12 months, unrealized losses | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember
12 months or longer, fair value | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember
12 months or longer, unrealized losses | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember
Total available-for-sale, fair value | 4 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember | 6 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember
Total available-for-sale, unrealized losses | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember
Fixed Maturities [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 814 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 845 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | (4)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (2)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 542 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 1,220 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (147)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (145)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 1,356 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 2,065 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (151)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (147)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | U S Treasury And Government [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 77 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 197 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 50 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 175 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (1)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (3)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 127 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 372 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (1)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (3)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | US States And Political Subdivisions [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 133 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 60 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | (1)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (1)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 73 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 257 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (3)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (7)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 206 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 317 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (4)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (8)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | Foreign Government Debt [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 22 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 20 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | (1)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 22 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 20 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (1)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | Corporate Obligations [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 363 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 468 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | (1)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (1)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 94 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 251 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (102)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (91)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 457 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 719 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (103)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (92)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | Residential Mortgage Backed Agency [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 154 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 16 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | (1)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 205 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 387 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (3)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (7)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 359 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 403 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (4)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (7)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | Residential Mortgage Backed Non Agency [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 8 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 10 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 18 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 19 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (4)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (4)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 26 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 29 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (4)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (4)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | Commercial Mortgage Backed Securities [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 2 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 4 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 1 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 6 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 3 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 10 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | Collateralized Debt Obligations [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 1 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 69 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 81 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (22)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (21)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 69 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 82 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (22)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (21)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Fixed Maturities [Member] | Other Asset Backed [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 55 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 69 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 32 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 44 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (12)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (12)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 87 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 113 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | $ (12)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | $ (12)us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember

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