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Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder (Benefits) And Claims Incurred Net | $ (6)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet | $ 50 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet |
Weighted average risk-free rate used to discount claim liability | 1.49%us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilityWeightedAverageRiskFreeDiscountRate |  |
Insurance loss recoverable | 527 mbi_InsuranceLossRecoverable |  | 533 mbi_InsuranceLossRecoverable
CDO Portfolio [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder (Benefits) And Claims Incurred Net | 2 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / mbi_ClassifiedStatusAxis = mbi_CdoPortfolioMember |  |
CDO Portfolio [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Elimination Of Loss And LAE Expense (Benefit) As Result Of Consolidating VIEs | 8 mbi_EliminationOfLossAndLaeExpenseAsResultOfConsolidatingVie / mbi_ClassifiedStatusAxis = mbi_CdoPortfolioMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |
Second Lien Residential Mortgage Backed Securities And Reserves [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder (Benefits) And Claims Incurred Net | (8)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember | 25 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember |
First Lien Residential Mortgage Backed Securities [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder (Benefits) And Claims Incurred Net | 9 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_FirstLienResidentialMortgageBackedSecuritiesMember | 30 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_FirstLienResidentialMortgageBackedSecuritiesMember |
Financial Guarantee [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Aggregate CDO Loss And LAE Reserves | 113 mbi_AggregateCdoLossAndLaeReserves / mbi_ClaimLiabilityAxis = us-gaap_FinancialGuaranteeMember |  | 111 mbi_AggregateCdoLossAndLaeReserves / mbi_ClaimLiabilityAxis = us-gaap_FinancialGuaranteeMember
Financial Guarantee [Member] | CDO Portfolio [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Elimination Of Loss And Lae Reserves As Result Of Consolidating VIEs | 233 mbi_Eliminationoflossandlaereservesasresultofconsolidatingvie / mbi_ClaimLiabilityAxis = us-gaap_FinancialGuaranteeMember / mbi_ClassifiedStatusAxis = mbi_CdoPortfolioMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  | 225 mbi_Eliminationoflossandlaereservesasresultofconsolidatingvie / mbi_ClaimLiabilityAxis = us-gaap_FinancialGuaranteeMember / mbi_ClassifiedStatusAxis = mbi_CdoPortfolioMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Loss And Lae Reserves [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Reserves for expected future claim payments | 609 mbi_ReservesForExpectedFutureClaimPayments / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  | 653 mbi_ReservesForExpectedFutureClaimPayments / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember
Expected recoveries of future payments | 110 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  | 147 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember
Recoveries On Unpaid Losses [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Expected recoveries of future payments | 110 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |  | 147 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember
Insurance Loss Recoverable [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Insurance loss recoverable | 527 mbi_InsuranceLossRecoverable / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |  |
Insurance Loss Recoverable [Member] | Classified [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Insurance claim loss recoverable | 527 us-gaap_EstimatedInsuranceRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember |  | 533 us-gaap_EstimatedInsuranceRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember
Insurance loss recoverable | 527 mbi_InsuranceLossRecoverable / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember |  | 533 mbi_InsuranceLossRecoverable / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember
Loan Repurchase Commitments [Member] | Second Lien Residential Mortgage Backed Securities And Reserves [Member] | Ineligible Loan Recoveries [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Total estimated insurance recoveries | $ 385 mbi_TotalEstimatedInsuranceRecoveries / mbi_ClaimLiabilityAxis = mbi_LoanRepurchaseCommitmentsMember / mbi_ClassifiedStatusAxis = mbi_IneligibleLoanRecoveriesMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember |  | $ 379 mbi_TotalEstimatedInsuranceRecoveries / mbi_ClaimLiabilityAxis = mbi_LoanRepurchaseCommitmentsMember / mbi_ClassifiedStatusAxis = mbi_IneligibleLoanRecoveriesMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember

X
- Definition Aggregate CDO loss and LAE reserves. + References No definition available. + Details
Name: | mbi_AggregateCdoLossAndLaeReserves
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Elimination Of Loss And LAE Expense As Result Of Consolidating VIEs + References No definition available. + Details
Name: | mbi_EliminationOfLossAndLaeExpenseAsResultOfConsolidatingVie
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Elimination Of Loss And Lae Reserves As Result Of Consolidating Vie + References No definition available. + Details
Name: | mbi_Eliminationoflossandlaereservesasresultofconsolidatingvie
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Gross reserve, recoveries on unpaid losses + References No definition available. + Details
Name: | mbi_GrossReserveRecoveriesOnUnpaidLosses
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Carrying amount as of the balance sheet date of paid insurance policy claims and LAE expense expected to be recovered. + References No definition available. + Details
Name: | mbi_InsuranceLossRecoverable
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | mbi_LossAndLossAdjustmentExpenseLineItems
Namespace Prefix: | mbi_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Reserves For Expected Future Claim Payments + References No definition available. + Details
Name: | mbi_ReservesForExpectedFutureClaimPayments
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Total Estimated Insurance Recoveries + References No definition available. + Details
Name: | mbi_TotalEstimatedInsuranceRecoveries
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Carrying amount as of the balance sheet date of amounts expected to be recovered under the terms of insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.17) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 17 -Article 5 + Details
Name: | us-gaap_EstimatedInsuranceRecoveries
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition This element represents the weighted average risk free rate used to discount the claim liability related to financial guarantee insurance contracts. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 9 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=6933314&loc=SL5749324-161292 + Details
Name: | us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilityWeightedAverageRiskFreeDiscountRate
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 5 -Article 7 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387 + Details
Name: | us-gaap_PolicyholderBenefitsAndClaimsIncurredNet
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | mbi_ClaimLiabilityAxis=mbi_InsuranceLossRecoverableMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | mbi_ClaimLiabilityAxis=mbi_LoanRepurchaseCommitmentsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | mbi_ClaimLiabilityAxis=mbi_LossAndLaeReservesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | mbi_ClaimLiabilityAxis=mbi_RecoveriesOnUnpaidLossesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | mbi_ClaimLiabilityAxis=us-gaap_FinancialGuaranteeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | mbi_ClassifiedStatusAxis=mbi_CdoPortfolioMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=mbi_FirstLienResidentialMortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |