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Loss And Loss Adjustment Expense Reserves (RMBS Reserves) (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] |  |  |  |  |
Insurance loss recoverable | $ 527 mbi_InsuranceLossRecoverable |  |  | $ 533 mbi_InsuranceLossRecoverable |
Schedule Of Insured Financial Obligations With Credit Deterioration Insured Contractual Payments Outstanding Principal | 10,884 us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal | [1] |  | 11,263 us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal | [1]
Policyholder (Benefits) And Claims Incurred Net | (6)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet |  | 50 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet |  |
Contract claims on ineligible mortgage loans inception to date, incurred loss on transactions | 418 mbi_ContractClaimsOnIneligibleMortgageLoansInceptionToDateIncurredLossOnTransactions |  |  |  |
Loss And Lae Reserves [Member] |  |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |  |
Loss and loss adjustment expense reserves | 499 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  |  | 506 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |
Expected recoveries of future payments | 110 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  |  | 147 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |
First Lien R M B S Reserves [Member] | Loss And Lae Reserves [Member] |  |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |  |
Loss and loss adjustment expense reserves | 267 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember / us-gaap_StatementBusinessSegmentsAxis = mbi_FirstLienRMBSReservesMember |  |  | 263 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember / us-gaap_StatementBusinessSegmentsAxis = mbi_FirstLienRMBSReservesMember |
Second Lien Rmbs And Heloc And Ces [Member] |  |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |  |
Addition to case basis reserves if losses remain at peak levels | 40 mbi_IncreaseDecreaseToCaseBasisReserves / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember |  |  |  |
Second Lien Rmbs And Heloc And Ces [Member] | Excess Spread [Member] |  |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |  |
Insurance loss recoverable | 488 mbi_InsuranceLossRecoverable / mbi_LoanStatusAxis = mbi_ExcessSpreadMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember |  |  | 496 mbi_InsuranceLossRecoverable / mbi_LoanStatusAxis = mbi_ExcessSpreadMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember |
Total excess spread recoveries | 502 us-gaap_EstimatedInsuranceRecoveries / mbi_LoanStatusAxis = mbi_ExcessSpreadMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember |  |  | 523 us-gaap_EstimatedInsuranceRecoveries / mbi_LoanStatusAxis = mbi_ExcessSpreadMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember |
Expected recoveries of future payments | 14 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_LoanStatusAxis = mbi_ExcessSpreadMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember |  |  | 27 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_LoanStatusAxis = mbi_ExcessSpreadMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember |
Second Lien Rmbs And Heloc And Ces [Member] | Excess Spread [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |  |
Insurance loss recoverable | 122 mbi_InsuranceLossRecoverable / mbi_LoanStatusAxis = mbi_ExcessSpreadMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  | 132 mbi_InsuranceLossRecoverable / mbi_LoanStatusAxis = mbi_ExcessSpreadMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Total excess spread recoveries | 124 us-gaap_EstimatedInsuranceRecoveries / mbi_LoanStatusAxis = mbi_ExcessSpreadMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  | 137 us-gaap_EstimatedInsuranceRecoveries / mbi_LoanStatusAxis = mbi_ExcessSpreadMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
Expected recoveries of future payments | 2 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_LoanStatusAxis = mbi_ExcessSpreadMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  | 5 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_LoanStatusAxis = mbi_ExcessSpreadMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsAndHelocAndCesMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
U S Public Finance Insurance [Member] |  |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |  |
Loss and loss adjustment expense reserves | 35 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = mbi_USPublicFinanceInsuranceMember |  |  | 45 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = mbi_USPublicFinanceInsuranceMember |
Insurance loss recoverable | 4 mbi_InsuranceLossRecoverable / us-gaap_StatementBusinessSegmentsAxis = mbi_USPublicFinanceInsuranceMember |  |  | 4 mbi_InsuranceLossRecoverable / us-gaap_StatementBusinessSegmentsAxis = mbi_USPublicFinanceInsuranceMember |
Policyholder (Benefits) And Claims Incurred Net | (6)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_USPublicFinanceInsuranceMember |  | (14)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_USPublicFinanceInsuranceMember |  |
U S Public Finance Insurance [Member] | General obligations |  |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |  |
Schedule Of Insured Financial Obligations With Credit Deterioration Insured Contractual Payments Outstanding Principal | 92,500 us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember / us-gaap_StatementBusinessSegmentsAxis = mbi_USPublicFinanceInsuranceMember |  |  | 98,900 us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember / us-gaap_StatementBusinessSegmentsAxis = mbi_USPublicFinanceInsuranceMember |
U S Public Finance Insurance [Member] | General obligations | Classified [Member] |  |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |  |
Schedule Of Insured Financial Obligations With Credit Deterioration Insured Contractual Payments Outstanding Principal | 83 us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember / us-gaap_StatementBusinessSegmentsAxis = mbi_USPublicFinanceInsuranceMember |  |  | 152 us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember / us-gaap_InvestmentTypeAxis = us-gaap_MunicipalBondsMember / us-gaap_StatementBusinessSegmentsAxis = mbi_USPublicFinanceInsuranceMember |
Second Lien R M B S Securitizations [Member] | Loss And Lae Reserves [Member] |  |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |  |
Loss and loss adjustment expense reserves | 71 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRMBSSecuritizationsMember |  |  | 70 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRMBSSecuritizationsMember |
Second Lien R M B S Securitizations [Member] | Variable Interest Entity Primary Beneficiary [Member] |  |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |  |
Loss and loss adjustment expense reserves | $ 29 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRMBSSecuritizationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |  |  | $ 21 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRMBSSecuritizationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember |
[1] | Represents contractual principal and interest payments due by the issuer of the obligations insured by MBIA.

X
- Definition Contract Claims On Ineligible Mortgage Loans Inception To Date Incurred Loss On Transactions + References No definition available. + Details
Name: | mbi_ContractClaimsOnIneligibleMortgageLoansInceptionToDateIncurredLossOnTransactions
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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 Increase Decrease To Case Basis Reserves + References No definition available. + Details
Name: | mbi_IncreaseDecreaseToCaseBasisReserves
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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

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 The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 3 -Subparagraph (a),(b),(c) -URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437 + Details
Name: | us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

X
- Definition This element represents the amount of insured contractual payments outstanding attributable to the principal of the underlying insured obligations for financial guarantee insurance contracts presented in the context of each surveillance grouping or category used by management to monitor and track potential deterioration in the credit worthiness of the insured party's ability to meet its financial obligations related to the contracts underlying financial guarantee insurance contracts the entity has issued. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 55 -Paragraph 32 -URI http://asc.fasb.org/extlink&oid=34726416&loc=SL5751133-161288 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 40 -Section 50 -Paragraph 9 -Subparagraph (a)(5)(iii) -URI http://asc.fasb.org/extlink&oid=6933314&loc=SL5749324-161292 + Details
Name: | us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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

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

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

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