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Fair Value Of Financial Instruments (Changes In Level 3 Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) (Fair Value Inputs Level 3 [Member], USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | $ 2,352 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue |  | $ 3,532 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue |
Realized gains/(losses), assets | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 |  | (9)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 |
Unrealized gains/(losses) included in earnings, assets | 15 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss |  | 342 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss |
Unrealized gains/(losses) included in OCI, assets | 31 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss |  | 36 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss |
Foreign exchange recognized in OCI or earnings, assets | (1)mbi_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisInForeignExchangeRecognizedOtherComprehensiveIncomeOrEarningsAssets |  | 0 mbi_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisInForeignExchangeRecognizedOtherComprehensiveIncomeOrEarningsAssets |
Purchases, assets | 26 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases |  | 75 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases |
Issuances, assets | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetIssues |
Settlements, assets | (316)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements |  | (1,272)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements |
Sales, assets | (110)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales |  | (374)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales |
Transfers into level 3, assets | 240 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 |  | 218 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 |
Transfers out of level 3, assets | (140)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 |  | (196)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 |
Ending balance, fair value assets | 2,097 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue |  | 2,352 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets | 51 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease |  | 350 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease |
Beginning balance, fair value liabilities | 2,301 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue |  | 5,039 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue |
Realized gains/(losses), liabilities | 445 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings |  | 1,548 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings |
Unrealized gains/(losses included in earnings, liabilities | (848)us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss |  | (1,577)us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss |
Unrealized gains/(losses) included in OCI, liabilities | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInOtherComprehensiveIncome |
Foreign exchange recognized in OCI or earnings, liabilities | (32)mbi_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisInForeignExchangeRecognizedOtherComprehensiveIncomeOrEarningsLiabilities |  | 9 mbi_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisInForeignExchangeRecognizedOtherComprehensiveIncomeOrEarningsLiabilities |
Purchases, liabilities | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPurchases |
Issuances, liabilities | 3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues |
Settlements, liabilities | (626)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements |  | (1,825)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements |
Sales, liabilities | (43)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales | [1] | (1,076)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales | [1]
Transfers into Level 3, liabilities | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3 |  | 185 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3 |
Transfers out of Level 3, liabilities | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3 |  | (2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3 |
Ending balance, fair value liabilities | 1,200 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue |  | 2,301 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease |  | 189 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease |
Loans Receivable [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 1,612 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 1,881 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Unrealized gains/(losses) included in earnings, assets | 39 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 180 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Settlements, assets | (220)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (275)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Sales, assets |  |  | (174)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Ending balance, fair value assets | 1,431 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 1,612 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets | 39 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 166 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = us-gaap_LoansReceivableMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Loan Repurchase Commitments [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 359 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 1,086 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Unrealized gains/(losses) included in earnings, assets | 20 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 172 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Settlements, assets |  |  | (899)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Ending balance, fair value assets | 379 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 359 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets | 20 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 172 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis = mbi_LoanRepurchaseCommitmentsMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
State and municipal bonds [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 19 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | 103 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Realized gains/(losses), assets |  |  | 2 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Unrealized gains/(losses) included in OCI, assets | 3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Settlements, assets | (4)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Sales, assets | (4)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | (13)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Transfers into level 3, assets | 46 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | 42 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Transfers out of level 3, assets | (52)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | (109)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Ending balance, fair value assets | 8 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |  | 19 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember |
Foreign Government Debt [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 12 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember |  | 3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember |
Purchases, assets |  |  | 10 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember |
Settlements, assets | (15)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember |  | (16)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember |
Transfers into level 3, assets | 9 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember |  | 18 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember |
Transfers out of level 3, assets |  |  | (3)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember |
Ending balance, fair value assets | 6 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember |  | 12 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_ForeignGovernmentDebtMember |
Corporate Obligations [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 48 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |  | 76 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |
Realized gains/(losses), assets | 2 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |  | (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |
Unrealized gains/(losses) included in earnings, assets | 3 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |  | 7 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |
Unrealized gains/(losses) included in OCI, assets | (3)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |  | 13 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |
Foreign exchange recognized in OCI or earnings, assets | (1)mbi_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisInForeignExchangeRecognizedOtherComprehensiveIncomeOrEarningsAssets / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |  |  |
Purchases, assets | 10 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |  | 1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |
Settlements, assets | (7)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |  | (16)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |
Sales, assets | (48)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |  | (28)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |
Transfers into level 3, assets | 7 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |  | 1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |
Transfers out of level 3, assets | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |  | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |
Ending balance, fair value assets | 10 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |  | 48 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |  | 7 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember |
Corporate Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 48 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 78 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Realized gains/(losses), assets |  |  | (4)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Unrealized gains/(losses) included in earnings, assets | (10)us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (7)us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Unrealized gains/(losses) included in OCI, assets |  |  | 6 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Settlements, assets | (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (4)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Sales, assets | (3)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (24)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Transfers into level 3, assets | 25 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Ending balance, fair value assets | 55 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 48 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CorporateObligationMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Residential Mortgage Backed Agency [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember |
Settlements, assets | (8)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember |  | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember |
Transfers into level 3, assets | 37 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember |  | 20 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember |
Transfers out of level 3, assets | (29)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember |  | (19)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember |
Ending balance, fair value assets | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember |  | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedAgencyMember |
Residential Mortgage Backed Non Agency [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 6 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember |  | 4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember |
Unrealized gains/(losses) included in OCI, assets | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember |  |  |
Settlements, assets |  |  | (2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember |
Transfers into level 3, assets |  |  | 5 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember |
Transfers out of level 3, assets | (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember |  | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember |
Ending balance, fair value assets | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember |  | 6 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember |
Residential Mortgage Backed Non Agency [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 6 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Unrealized gains/(losses) included in earnings, assets | (1)us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 6 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Settlements, assets | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (7)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Transfers into level 3, assets | 2 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Transfers out of level 3, assets | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (4)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Ending balance, fair value assets | 3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_ResidentialMortgageBackedNonAgencyMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Commercial Mortgage Backed Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 14 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  | 28 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |
Unrealized gains/(losses) included in OCI, assets |  |  | 4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |
Settlements, assets | (14)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |
Sales, assets |  |  | (19)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |
Transfers into level 3, assets | 3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  | 3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |
Transfers out of level 3, assets | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |
Ending balance, fair value assets | 2 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |  | 14 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember |
Commercial Mortgage Backed Securities [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 7 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Unrealized gains/(losses) included in earnings, assets | (3)us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 0 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Sales, assets |  |  | (24)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Transfers into level 3, assets |  |  | 20 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Ending balance, fair value assets | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets |  |  | 1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CommercialMortgageBackedSecuritiesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Collateralized Debt Obligations [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 82 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |  | 31 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |
Realized gains/(losses), assets | (2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |  | (2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInEarnings1 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |
Unrealized gains/(losses) included in earnings, assets | 1 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |  | 1 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |
Unrealized gains/(losses) included in OCI, assets | 28 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |  | 12 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |
Purchases, assets | 5 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |  | 61 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |
Settlements, assets | (15)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |  | (17)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |
Sales, assets | (41)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |  | (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |
Transfers into level 3, assets | 36 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |  | 31 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |
Transfers out of level 3, assets | (7)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |  | (30)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |
Ending balance, fair value assets | 87 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |  | 82 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets |  |  | 1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember |
Collateralized Debt Obligations [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 22 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 125 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Unrealized gains/(losses) included in earnings, assets | (10)us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (8)us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Unrealized gains/(losses) included in OCI, assets |  |  | 5 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Settlements, assets | (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Sales, assets |  |  | (85)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Transfers into level 3, assets | 1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Transfers out of level 3, assets | (3)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (14)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Ending balance, fair value assets | 5 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 22 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets | 1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_CollateralizedDebtObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Other Asset Backed [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 58 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |  | 26 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |
Unrealized gains/(losses) included in OCI, assets | 4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |  | (3)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetGainLossIncludedInOtherComprehensiveIncomeLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |
Purchases, assets | 11 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |  | 3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPurchases / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |
Settlements, assets | (12)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |  | (10)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |
Sales, assets | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |  |  |
Transfers into level 3, assets | 52 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |  | 47 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |
Transfers out of level 3, assets | (27)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |  | (5)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |
Ending balance, fair value assets | 85 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |  | 58 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember |
Other Asset Backed [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 54 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 64 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Unrealized gains/(losses) included in earnings, assets | (26)us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (9)us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Settlements, assets | (10)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (14)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Sales, assets |  |  | (2)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Transfers into level 3, assets | 18 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 21 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Transfers out of level 3, assets | (10)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | (6)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Ending balance, fair value assets | 26 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 54 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets | (9)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 2 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherAssetBackedMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Medium Term Notes [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value liabilities | 203 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_MediumTermNotesMember |  | 165 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_MediumTermNotesMember |
Unrealized gains/(losses included in earnings, liabilities | 20 us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_MediumTermNotesMember |  | 29 us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_MediumTermNotesMember |
Foreign exchange recognized in OCI or earnings, liabilities | (26)mbi_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisInForeignExchangeRecognizedOtherComprehensiveIncomeOrEarningsLiabilities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_MediumTermNotesMember |  | 9 mbi_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisInForeignExchangeRecognizedOtherComprehensiveIncomeOrEarningsLiabilities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_MediumTermNotesMember |
Ending balance, fair value liabilities | 197 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_MediumTermNotesMember |  | 203 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_MediumTermNotesMember |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities | (6)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_MediumTermNotesMember |  | 38 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = us-gaap_MediumTermNotesMember |
Credit Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value liabilities | 1,147 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |  | 2,921 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |
Realized gains/(losses), liabilities | 415 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |  | 1,548 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |
Unrealized gains/(losses included in earnings, liabilities | (903)us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |  | (1,778)us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |
Settlements, liabilities | (415)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |  | (1,548)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |
Transfers into Level 3, liabilities |  |  | 4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |
Ending balance, fair value liabilities | 244 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |  | 1,147 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities | (20)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |  | 87 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CreditDerivativesMember |
Interest Rate Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value liabilities |  |  | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_InterestRateDerivativesMember |
Unrealized gains/(losses included in earnings, liabilities |  |  | 2 us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_InterestRateDerivativesMember |
Transfers out of Level 3, liabilities |  |  | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_InterestRateDerivativesMember |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities |  |  | (18)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_InterestRateDerivativesMember |
Other Derivative Liabilities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value liabilities | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember |  |  |
Realized gains/(losses), liabilities | 30 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityGainLossIncludedInEarnings / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember |  |  |
Unrealized gains/(losses included in earnings, liabilities | (6)us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember |  |  |
Ending balance, fair value liabilities | 24 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember |  |  |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities | (6)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_OtherDerivativeLiabilitiesMember |  |  |
Variable Interest Entity Notes [Member] | Liabilities Of Consolidated Vies [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value liabilities | 940 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |  | 1,932 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |
Unrealized gains/(losses included in earnings, liabilities | 46 us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |  | 180 us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |
Issuances, liabilities | 3 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIssues / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |  |  |
Settlements, liabilities | (211)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |  | (277)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySettlements / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |
Sales, liabilities | (43)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |  | (1,076)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilitySales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |
Transfers into Level 3, liabilities |  |  | 181 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |
Ending balance, fair value liabilities | 735 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |  | 940 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities | 42 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |  | 92 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_VariableInterestEntityNotesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |
Currency Derivatives [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value liabilities |  |  | 1 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CurrencyDerivativesMember |
Transfers out of Level 3, liabilities |  |  | (1)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationLiabilityTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CurrencyDerivativesMember |
Currency Derivatives [Member] | Liabilities Of Consolidated Vies [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value liabilities | 11 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CurrencyDerivativesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |  | 21 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CurrencyDerivativesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |
Unrealized gains/(losses included in earnings, liabilities | (5)us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CurrencyDerivativesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |  | (10)us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CurrencyDerivativesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |
Foreign exchange recognized in OCI or earnings, liabilities | (6)mbi_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisInForeignExchangeRecognizedOtherComprehensiveIncomeOrEarningsLiabilities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CurrencyDerivativesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |  | 0 mbi_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisInForeignExchangeRecognizedOtherComprehensiveIncomeOrEarningsLiabilities / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CurrencyDerivativesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |
Ending balance, fair value liabilities | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CurrencyDerivativesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |  | 11 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CurrencyDerivativesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |
Change in unrealized gains/(losses) for the period included in earnings for liabilities still held, liabilities | (11)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CurrencyDerivativesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |  | (10)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_CurrencyDerivativesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = mbi_LiabilitiesOfConsolidatedViesMember |
Perpetual Debt And Equity Securities [Member] |  |  |  |
Accounts Notes And Loans Receivable [Line Items] |  |  |  |
Beginning balance, fair value assets | 11 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  | 14 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |
Unrealized gains/(losses) included in earnings, assets | 2 us-gaap_FairValueAssetsMeasuredOnRecurringBasisChangeInUnrealizedGainLoss / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  |  |
Sales, assets | (13)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetSales / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  |  |
Transfers into level 3, assets | 4 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersIntoLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  |  |
Transfers out of level 3, assets | (4)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  | (3)us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetTransfersOutOfLevel3 / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |
Ending balance, fair value assets | 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  | 11 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetValue / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |
Change in unrealized gains/(losses) for the period included in earnings for assets still held, assets | $ 0 us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisAssetPeriodIncreaseDecrease / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / invest_InvestmentAxis = mbi_PerpetualDebtAndEquitySecuritiesMember |  |  |
[1] | Primarily relates to the deconsolidation of VIEs.

X
- Definition Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis In Foreign Exchange Recognized Other Comprehensive Income or Earnings, Assets + References No definition available. + Details
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- Definition Fair Value Measurement With Unobservable Inputs Reconciliation Recurring Basis In Foreign Exchange Recognized Other Comprehensive Income or Earnings, Liabilities + References No definition available. + Details
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- Definition Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those assets still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Amount of unrealized (holding) gain (loss) which is included in the statement of income (or changes in net assets) related to those liabilities still held at the reporting date for which fair value is measured on a recurring basis using significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Amount of transfers of financial instrument classified as a liability into level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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- Definition Amount of transfers of financial instrument classified as a liability out of level 3 of the fair value hierarchy. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=25499696&loc=d3e19207-110258 + Details
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