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Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder (Benefits) And Claims Incurred Net | $ 133,000,000 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet | $ 117,000,000 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet | $ 50,000,000 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet
Weighted average risk-free rate used to discount claim liability | 1.72%us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilityWeightedAverageRiskFreeDiscountRate | 2.37%us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilityWeightedAverageRiskFreeDiscountRate |
CDO Portfolio [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Policyholder (Benefits) And Claims Incurred Net | 33,000,000 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 | 1,000,000 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 | 70,000,000 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember | (2,000,000)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember | (151,000,000)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember
Elimination Of Loss And LAE Expense (Benefit) As Result Of Consolidating VIEs | 20,000,000 mbi_EliminationOfLossAndLaeExpenseAsResultOfConsolidatingVie / 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 | 65,000,000 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_FirstLienResidentialMortgageBackedSecuritiesMember | (70,000,000)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_FirstLienResidentialMortgageBackedSecuritiesMember | 147,000,000 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_FirstLienResidentialMortgageBackedSecuritiesMember
Elimination Of Loss And LAE Expense (Benefit) As Result Of Consolidating VIEs | (2,000,000)mbi_EliminationOfLossAndLaeExpenseAsResultOfConsolidatingVie / us-gaap_StatementBusinessSegmentsAxis = mbi_FirstLienResidentialMortgageBackedSecuritiesMember |  |
Financial Guarantee [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Aggregate CDO Loss And LAE Reserves | 111,000,000 mbi_AggregateCdoLossAndLaeReserves / mbi_ClaimLiabilityAxis = us-gaap_FinancialGuaranteeMember | 127,000,000 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 | 225,000,000 mbi_Eliminationoflossandlaereservesasresultofconsolidatingvie / mbi_ClaimLiabilityAxis = us-gaap_FinancialGuaranteeMember / mbi_ClassifiedStatusAxis = mbi_CdoPortfolioMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 226,000,000 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 | 653,000,000 mbi_ReservesForExpectedFutureClaimPayments / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember | 847,000,000 mbi_ReservesForExpectedFutureClaimPayments / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |
Expected recoveries of future payments | 147,000,000 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember | 206,000,000 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |
Recoveries On Unpaid Losses [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Expected recoveries of future payments | 147,000,000 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember | 206,000,000 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember | 332,000,000 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember
Insurance Loss Recoverable [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Insurance loss recoverable | 533,000,000 mbi_InsuranceLossRecoverable / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember | 694,000,000 mbi_InsuranceLossRecoverable / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember | 3,648,000,000 mbi_InsuranceLossRecoverable / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember
Insurance Loss Recoverable [Member] | Classified [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Insurance claim loss recoverable | 533,000,000 us-gaap_EstimatedInsuranceRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember | 694,000,000 us-gaap_EstimatedInsuranceRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember |
Insurance loss recoverable | 533,000,000 mbi_InsuranceLossRecoverable / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember | 694,000,000 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 | $ 379,000,000 mbi_TotalEstimatedInsuranceRecoveries / mbi_ClaimLiabilityAxis = mbi_LoanRepurchaseCommitmentsMember / mbi_ClassifiedStatusAxis = mbi_IneligibleLoanRecoveriesMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember | $ 359,000,000 mbi_TotalEstimatedInsuranceRecoveries / mbi_ClaimLiabilityAxis = mbi_LoanRepurchaseCommitmentsMember / mbi_ClassifiedStatusAxis = mbi_IneligibleLoanRecoveriesMember / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember |

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

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- Definition Elimination Of Loss And LAE Expense As Result Of Consolidating VIEs + References No definition available. + Details
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- Definition Elimination Of Loss And Lae Reserves As Result Of Consolidating Vie + References No definition available. + Details
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- 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

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- 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
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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

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- 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

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- 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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- 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
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Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- 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
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Namespace Prefix: | us-gaap_
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- Details
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Data Type: | na
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- Details
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Namespace Prefix: |
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- Details
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