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Loss And Loss Adjustment Expense Reserves (RMBS Reserves) (Narrative) (Detail) (USD $)
12 Months Ended 6 Months Ended 0 Months Ended 18 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Nov. 30, 2014
May 31, 2015
Dec. 31, 2015
May 31, 2016
Loss And Loss Adjustment Expense Reserves [Line Items]              
Schedule Of Insured Financial Obligations With Credit Deterioration Insured Contractual Payments Outstanding Principal $ 11,263,000,000us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal [1] $ 14,264,000,000us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal [1]          
Policyholder (Benefits) And Claims Incurred Net 133,000,000us-gaap_PolicyholderBenefitsAndClaimsIncurredNet 117,000,000us-gaap_PolicyholderBenefitsAndClaimsIncurredNet 50,000,000us-gaap_PolicyholderBenefitsAndClaimsIncurredNet        
Loss And Lae Reserves [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Loss and loss adjustment expense reserves 506,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ mbi_ClaimLiabilityAxis
= mbi_LossAndLaeReservesMember
641,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ mbi_ClaimLiabilityAxis
= mbi_LossAndLaeReservesMember
853,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ mbi_ClaimLiabilityAxis
= mbi_LossAndLaeReservesMember
       
Expected recoveries of future payments 147,000,000mbi_GrossReserveRecoveriesOnUnpaidLosses
/ mbi_ClaimLiabilityAxis
= mbi_LossAndLaeReservesMember
206,000,000mbi_GrossReserveRecoveriesOnUnpaidLosses
/ mbi_ClaimLiabilityAxis
= mbi_LossAndLaeReservesMember
         
Classified [Member] | Loss And Lae Reserves [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Loss and loss adjustment expense reserves 506,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ mbi_ClaimLiabilityAxis
= mbi_LossAndLaeReservesMember
/ mbi_ClassifiedStatusAxis
= mbi_ClassifiedMember
641,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ mbi_ClaimLiabilityAxis
= mbi_LossAndLaeReservesMember
/ mbi_ClassifiedStatusAxis
= mbi_ClassifiedMember
         
First Lien R M B S Reserves [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
90+ day roll rate to loss set stress scenario       90.00%mbi_PercentageOfRollRateToLossSet
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_FirstLienRMBSReservesMember
     
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 263,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ mbi_ClaimLiabilityAxis
= mbi_LossAndLaeReservesMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_FirstLienRMBSReservesMember
241,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ mbi_ClaimLiabilityAxis
= mbi_LossAndLaeReservesMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_FirstLienRMBSReservesMember
         
First Lien R M B S Reserves [Member] | Variable Interest Entity Primary Beneficiary [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Loss and loss adjustment expense reserves 2,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_FirstLienRMBSReservesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
2,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_FirstLienRMBSReservesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
         
First Lien R M B S Reserves [Member] | ALT-A [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Percentage of roll to loss after declining over a 24-month period       25.00%mbi_PercentageOfLossAfterDecliningOverTwentyFourMonthPeriod
/ mbi_DelinquencyPeriodAxis
= mbi_AltMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_FirstLienRMBSReservesMember
     
First Lien R M B S Reserves [Member] | Foreclosure [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Assumed roll rate for loans       90.00%mbi_AssumedRollRateForLoans
/ mbi_LoanStatusAxis
= mbi_ForeclosureMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_FirstLienRMBSReservesMember
     
First Lien R M B S Reserves [Member] | Real Estate Owned [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Assumed roll rate for loans       90.00%mbi_AssumedRollRateForLoans
/ mbi_LoanStatusAxis
= mbi_RealEstateOwnedMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_FirstLienRMBSReservesMember
     
First Lien R M B S Reserves [Member] | Bankruptcy [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Assumed roll rate for loans       75.00%mbi_AssumedRollRateForLoans
/ mbi_LoanStatusAxis
= mbi_BankruptcyMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_FirstLienRMBSReservesMember
     
Rmbs And Heloc And Ces [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Assumed roll rate for ninety plus day delinquent loans excluding foreclosure and real estate owned       95.00%mbi_AssumedRollRateForNinetyPlusDayDelinquentLoansExcludingForeclosureAndRealEstateOwned
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
     
Period of elevated delinquency and loss for base case, beginning, months         6mbi_PeriodOfElevatedDelinquencyAndLossForBaseCaseBeginningMonths
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
   
Percentage of loans in 30-59 days delinquent bucket       10.00%mbi_PercentageOfLoansInThirtyToFiftyNineDayDelinquentBucket
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
     
Percentage of loans charged-off       30.00%mbi_PercentageOfLoansChargedOff
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
     
Percentage of Current Roll to Loss for the transaction       3.00%mbi_PercentageOfCurrentRollToLossForTransaction
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
     
Addition to case basis reserves if losses remain at peak levels 45,000,000mbi_IncreaseDecreaseToCaseBasisReserves
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
           
Rmbs And Heloc And Ces [Member] | Scenario Forecast [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Decrease in percentage of Current Roll to Loss for transaction         25.00%mbi_DecreaseInPercentageOfCurrentRollToLossForTransaction
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
   
Current Roll to Loss           0.00%mbi_PercentageOfFurtherReductionOfCurrentRollToLossForTransaction
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
 
Rmbs And Heloc And Ces [Member] | Scenario Forecast [Member] | Minimum [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Period Of Current Roll To Loss For Stress Case Additional Months         6mbi_Periodofcurrentrolltolossforstresscaseadditionalmonths
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
   
Rmbs And Heloc And Ces [Member] | Scenario Forecast [Member] | Maximum [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Period Of Current Roll To Loss For Stress Case Additional Months             18mbi_Periodofcurrentrolltolossforstresscaseadditionalmonths
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
Rmbs And Heloc And Ces [Member] | Scenario Forecast [Member] | Rate Reduction [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Decrease in percentage of Current Roll to Loss for transaction         0.75%mbi_DecreaseInPercentageOfCurrentRollToLossForTransaction
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
/ us-gaap_StatementScenarioAxis
= us-gaap_ScenarioForecastMember
/ us-gaap_ValuationAllowancesAndReservesTypeAxis
= mbi_RateReductionMember
   
Rmbs And Heloc And Ces [Member] | Excess Spread [Member]              
Loss And Loss Adjustment Expense Reserves [Line Items]              
Insurance loss recoverable 496,000,000mbi_InsuranceLossRecoverable
/ mbi_LoanStatusAxis
= mbi_ExcessSpreadMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
647,000,000mbi_InsuranceLossRecoverable
/ mbi_LoanStatusAxis
= mbi_ExcessSpreadMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
         
Total excess spread recoveries 523,000,000us-gaap_EstimatedInsuranceRecoveries
/ mbi_LoanStatusAxis
= mbi_ExcessSpreadMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
681,000,000us-gaap_EstimatedInsuranceRecoveries
/ mbi_LoanStatusAxis
= mbi_ExcessSpreadMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
         
Expected recoveries of future payments 27,000,000mbi_GrossReserveRecoveriesOnUnpaidLosses
/ mbi_LoanStatusAxis
= mbi_ExcessSpreadMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
34,000,000mbi_GrossReserveRecoveriesOnUnpaidLosses
/ mbi_LoanStatusAxis
= mbi_ExcessSpreadMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
         
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 132,000,000mbi_InsuranceLossRecoverable
/ mbi_LoanStatusAxis
= mbi_ExcessSpreadMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
183,000,000mbi_InsuranceLossRecoverable
/ mbi_LoanStatusAxis
= mbi_ExcessSpreadMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
         
Total excess spread recoveries 137,000,000us-gaap_EstimatedInsuranceRecoveries
/ mbi_LoanStatusAxis
= mbi_ExcessSpreadMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
190,000,000us-gaap_EstimatedInsuranceRecoveries
/ mbi_LoanStatusAxis
= mbi_ExcessSpreadMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
         
Expected recoveries of future payments 5,000,000mbi_GrossReserveRecoveriesOnUnpaidLosses
/ mbi_LoanStatusAxis
= mbi_ExcessSpreadMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
7,000,000mbi_GrossReserveRecoveriesOnUnpaidLosses
/ mbi_LoanStatusAxis
= mbi_ExcessSpreadMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_RmbsAndHelocAndCesMember
/ 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 45,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_USPublicFinanceInsuranceMember
87,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_USPublicFinanceInsuranceMember
         
Insurance loss recoverable 4,000,000mbi_InsuranceLossRecoverable
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_USPublicFinanceInsuranceMember
13,000,000mbi_InsuranceLossRecoverable
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_USPublicFinanceInsuranceMember
         
Policyholder (Benefits) And Claims Incurred Net (10,000,000)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 98,900,000,000us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal
/ us-gaap_InvestmentTypeAxis
= us-gaap_MunicipalBondsMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_USPublicFinanceInsuranceMember
124,900,000,000us-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 152,000,000us-gaap_ScheduleOfInsuredFinancialObligationsWithCreditDeteriorationInsuredContractualPaymentsOutstandingPrincipal
/ mbi_ClassifiedStatusAxis
= mbi_ClassifiedMember
/ us-gaap_InvestmentTypeAxis
= us-gaap_MunicipalBondsMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_USPublicFinanceInsuranceMember
161,000,000us-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 70,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ mbi_ClaimLiabilityAxis
= mbi_LossAndLaeReservesMember
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_SecondLienRMBSSecuritizationsMember
126,000,000us-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 $ 21,000,000us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
/ us-gaap_StatementBusinessSegmentsAxis
= mbi_SecondLienRMBSSecuritizationsMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
$ 43,000,000us-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.