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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 | Dec. 31, 2014 | Dec. 31, 2013
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Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | $ 851 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue | $ 2,978 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Less than 12 months, unrealized losses | (2)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments | (128)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments
12 months or longer, fair value | 1,220 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue | 479 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
12 months or longer, unrealized losses | (145)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments | (101)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments
Total available-for-sale, fair value | 2,071 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue | 3,457 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Total available-for-sale, unrealized losses | (147)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments | (229)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments
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 mbi_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss | 0 mbi_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
12 months or longer, fair value | 373 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue | 1,244 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
12 months or longer, unrealized losses | (202)mbi_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss | (181)mbi_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Total held-to-maturity, fair value | 373 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue | 1,244 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
Total held-to-maturity, unrealized losses | (202)mbi_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss | (181)mbi_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 mbi_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 0 mbi_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
12 months or longer, fair value | 373 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 1,244 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
12 months or longer, unrealized losses | (202)mbi_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | (181)mbi_HeldToMaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Total held-to-maturity, fair value | 373 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 1,244 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Total held-to-maturity, unrealized losses | (202)mbi_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | (181)mbi_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 | 6 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember | 5 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember
Less than 12 months, unrealized losses | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / 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 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember
Total available-for-sale, fair value | 6 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember | 5 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember
Total available-for-sale, unrealized losses | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember
Fixed Maturities [Member] |  |
Available For Sale Securities [Abstract] |  |
Less than 12 months, fair value | 845 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 2,973 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | (2)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (128)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 1,220 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 479 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (145)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (101)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 2,065 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 3,452 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (147)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (229)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / 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 | 197 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 269 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (9)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 175 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 1 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (3)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 372 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 270 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (3)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (9)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / 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 | 60 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 1,112 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | (1)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (65)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 257 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 49 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (7)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (8)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 317 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 1,161 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (8)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (73)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / 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 | 20 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 36 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / 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 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 20 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 36 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / 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 | 468 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 788 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | (1)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (30)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 251 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 82 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (91)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (8)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 719 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 870 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (92)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (38)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / 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 | 16 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 713 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (23)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 387 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 144 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (7)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (8)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 403 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 857 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (7)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (31)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / 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 | 10 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 17 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 19 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 22 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (4)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (5)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 29 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 39 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (4)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (5)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / 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 | 4 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 11 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (1)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 6 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 10 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 11 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (1)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / 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 | 1 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 6 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 81 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 124 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (21)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (57)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 82 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 130 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | (21)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (57)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / 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 | 69 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 21 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Less than 12 months, unrealized losses | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 0 mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, fair value | 44 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 57 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
12 months or longer, unrealized losses | (12)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | (15)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, fair value | 113 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | 78 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember
Total available-for-sale, unrealized losses | $ (12)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | $ (15)mbi_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAggregateLossesAccumulatedInInvestments / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember

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