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Investments (Amortized Cost And Fair Value Of Available-For-Sale and Held-To-Maturity Investment Portfolios) (Detail) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Available For Sale Securities [Abstract] |  |  |  |
Amortized cost | $ 6,509 us-gaap_AvailableForSaleSecuritiesAmortizedCost |  | $ 6,824 us-gaap_AvailableForSaleSecuritiesAmortizedCost |
Gross unrealized gains | 209 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments |  | 97 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments |
Gross unrealized losses | (147)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments |  | (229)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments |
Total available-for-sale, fair value | 6,571 us-gaap_AvailableForSaleSecurities |  | 6,692 us-gaap_AvailableForSaleSecurities |
Other-than-temporary impairments | (90)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities | [1] | (10)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities | [1]
Held To Maturity Securities [Abstract] |  |  |  |
Amortized cost | 2,757 us-gaap_HeldToMaturitySecurities |  | 2,801 us-gaap_HeldToMaturitySecurities |
Gross unrealized gains | 77 mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain |  | 31 mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain |
Gross unrealized losses | (202)mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss |  | (181)mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss |
Total Held-To-Maturity, fair value | 2,632 us-gaap_HeldToMaturitySecuritiesFairValue |  | 2,651 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 | 2,757 us-gaap_HeldToMaturitySecurities / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 2,801 us-gaap_HeldToMaturitySecurities / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Gross unrealized gains | 77 mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 31 mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingGain / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Gross unrealized losses | (202)mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (181)mbi_HeldToMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Total Held-To-Maturity, fair value | 2,632 us-gaap_HeldToMaturitySecuritiesFairValue / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 2,651 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 | 422 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember |  | 781 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember |
Gross unrealized gains | 0 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember |  | 0 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember |
Gross unrealized losses | 0 mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember |  | 0 mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember |
Total available-for-sale, fair value | 422 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember |  | 781 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]
Money Market Securities [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Available For Sale Securities [Abstract] |  |  |  |
Amortized cost |  |  | 136 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Gross unrealized gains |  |  | 0 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Gross unrealized losses |  |  | 0 mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Total available-for-sale, fair value |  |  | 136 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Other-than-temporary impairments |  |  | 0 mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities / invest_InvestmentAxis = mbi_MoneyMarketSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | [1]
Perpetual Debt And Equity Securities [Member] |  |  |  |
Available For Sale Securities [Abstract] |  |  |  |
Amortized cost | 12 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  | 10 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |
Gross unrealized gains | 1 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  | 1 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |
Gross unrealized losses | 0 mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  | 0 mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |
Total available-for-sale, fair value | 13 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  | 11 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 | 6,075 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 5,897 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized gains | 208 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 96 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized losses | (147)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | (229)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Total available-for-sale, fair value | 6,136 us-gaap_AvailableForSaleSecurities / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 5,764 us-gaap_AvailableForSaleSecurities / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Other-than-temporary impairments | (90)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | [1] | (10)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | [1]
Fixed Maturities [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Held To Maturity Securities [Abstract] |  |  |  |
Amortized cost | 2,757 us-gaap_HeldToMaturitySecurities / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  |
Total Held-To-Maturity, fair value | 2,632 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 | 631 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 528 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized gains | 39 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 16 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized losses | (3)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | (9)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Total available-for-sale, fair value | 667 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_USTreasuryAndGovernmentMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 535 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 |  | 1,831 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized gains | 94 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 22 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized losses | (8)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | (73)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Total available-for-sale, fair value | 1,730 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 1,780 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 | 283 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 184 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized gains | 7 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 5 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized losses | 0 mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 0 mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Total available-for-sale, fair value | 290 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 189 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 | 1,984 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 1,682 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized gains | 44 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 24 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized losses | (92)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | (38)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Total available-for-sale, fair value | 1,936 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 1,668 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Other-than-temporary impairments | (86)mbi_AvailableForSaleCumulativeOtherThanTemporaryImpairmentOfSecurities / invest_InvestmentAxis = us-gaap_CorporateDebtSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember | [1] | 0 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 | 1,116 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 1,167 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized gains | 17 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 10 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized losses | (7)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | (31)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Total available-for-sale, fair value | 1,126 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 1,146 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 | 54 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 79 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized gains | 3 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 13 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized losses | (4)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | (5)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Total available-for-sale, fair value | 53 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 87 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 | 19 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 34 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized gains | 1 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 1 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized losses | 0 mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | (1)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Total available-for-sale, fair value | 20 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 34 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 | 113 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 205 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized gains | 0 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 3 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized losses | (21)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | (57)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Total available-for-sale, fair value | 92 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 151 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] | (14)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 | 231 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 187 us-gaap_AvailableForSaleSecuritiesAmortizedCost / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized gains | 3 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 2 mbi_AvailableForSaleSecuritiesGrossUnrealizedGainAccumulatedInInvestments / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Gross unrealized losses | (12)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | (15)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |
Total available-for-sale, fair value | 222 us-gaap_AvailableForSaleSecurities / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = us-gaap_FixedMaturitiesMember |  | 174 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]
[1] | Represents unrealized gains or losses on other than temporarily impaired securities recognized in AOCI, which includes the non-credit component of impairments, as well as all subsequent changes in fair value of such impaired securities reported in AOCI.

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- Definition Unrealized gains or losses on other-than-temporarily impaired securities recognized in AOCI, which includes the non-credit component of impairments, as well as all subsequent changes in fair value of such impaired securities reported in AOCI. + References No definition available. + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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