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