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Fair Value Of Financial Instruments (Aggregate Fair Value And Remaining Contractual Principal Balance Outstanding On Fair Value Option) (Detail) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Non Variable Interest Entity [Member] |  |
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items] |  |
Medium-term notes, contractual outstanding principal | $ 215 mbi_Fairvalueoptioncontractualoutstandingprincipalmediumtermnotes / dei_LegalEntityAxis = us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember | $ 242 mbi_Fairvalueoptioncontractualoutstandingprincipalmediumtermnotes / dei_LegalEntityAxis = us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
Medium-term notes, fair value | 180 mbi_FairValueOptionAggregateFairValueMediumTermNotes / dei_LegalEntityAxis = us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember | 197 mbi_FairValueOptionAggregateFairValueMediumTermNotes / dei_LegalEntityAxis = us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
Medium-term notes, difference | 35 mbi_FairValueOptionAggregateDifferencesMediumTermNotes / dei_LegalEntityAxis = us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember | 45 mbi_FairValueOptionAggregateDifferencesMediumTermNotes / dei_LegalEntityAxis = us-gaap_ConsolidatedEntityExcludingVariableInterestEntitiesVIEMember
Variable Interest Entity [Member] |  |
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items] |  |
Loans receivable, contractual outstanding principal | 1,876 mbi_FairValueOptionContractualOutstandingPrincipalLoansReceivable / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 2,502 mbi_FairValueOptionContractualOutstandingPrincipalLoansReceivable / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Variable interest entity notes, contractual outstanding principal | 4,000 mbi_FairValueOptionContractualOutstandingPrincipalLongTermDebtInstruments / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 3,584 mbi_FairValueOptionContractualOutstandingPrincipalLongTermDebtInstruments / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Loans receivable, fair value | 1,480 mbi_FairValueOptionAggregateFairValueLoansReceivable / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 2,142 mbi_FairValueOptionAggregateFairValueLoansReceivable / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Variable interest entity notes, fair value | 2,653 mbi_FairValueOptionAggregateFairValueLongTermDebtInstruments / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 2,479 mbi_FairValueOptionAggregateFairValueLongTermDebtInstruments / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Loans receivable, difference | 396 us-gaap_FairValueOptionAggregateDifferencesLoansAndLongTermReceivables / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 360 us-gaap_FairValueOptionAggregateDifferencesLoansAndLongTermReceivables / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Variable interest entity notes, difference | 1,347 us-gaap_FairValueOptionAggregateDifferencesLongTermDebtInstruments / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember | 1,105 us-gaap_FairValueOptionAggregateDifferencesLongTermDebtInstruments / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Residential Mortgage Loans [Member] | Variable Interest Entity [Member] |  |
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items] |  |
Loans receivable, contractual outstanding principal | 1,478 mbi_FairValueOptionContractualOutstandingPrincipalLoansReceivable / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_ResidentialMortgageLoansMember | 1,554 mbi_FairValueOptionContractualOutstandingPrincipalLoansReceivable / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_ResidentialMortgageLoansMember
Loans receivable, 90 days or more past due, contractual outstanding principal | 220 us-gaap_FairValueOptionLoansHeldAsAssets90DaysOrMorePastDue / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_ResidentialMortgageLoansMember | 227 us-gaap_FairValueOptionLoansHeldAsAssets90DaysOrMorePastDue / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_ResidentialMortgageLoansMember
Loans receivable, fair value | 1,320 mbi_FairValueOptionAggregateFairValueLoansReceivable / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_ResidentialMortgageLoansMember | 1,377 mbi_FairValueOptionAggregateFairValueLoansReceivable / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_ResidentialMortgageLoansMember
Loans receivable, 90 days or more past due, fair value | 52 us-gaap_FairValueOptionLoansHeldAsAssetsAggregateAmountInNonaccrualStatus / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_ResidentialMortgageLoansMember | 54 us-gaap_FairValueOptionLoansHeldAsAssetsAggregateAmountInNonaccrualStatus / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_ResidentialMortgageLoansMember
Loans receivable, difference | 158 us-gaap_FairValueOptionAggregateDifferencesLoansAndLongTermReceivables / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_ResidentialMortgageLoansMember | 177 us-gaap_FairValueOptionAggregateDifferencesLoansAndLongTermReceivables / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_ResidentialMortgageLoansMember
Loans receivable, 90 days or more past due, difference | 168 us-gaap_FairValueOptionLoansHeldAsAssets90DaysOrMorePastDueAggregateDifference / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_ResidentialMortgageLoansMember | 173 us-gaap_FairValueOptionLoansHeldAsAssets90DaysOrMorePastDueAggregateDifference / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_ResidentialMortgageLoansMember
Other Loans [Member] | Variable Interest Entity [Member] |  |
Schedule Of Fair Value Of Separate Accounts By Major Category Of Investment [Line Items] |  |
Loans receivable, contractual outstanding principal | 108 mbi_FairValueOptionContractualOutstandingPrincipalLoansReceivable / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_OtherLoansMember | 721 mbi_FairValueOptionContractualOutstandingPrincipalLoansReceivable / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_OtherLoansMember
Loans receivable, 90 days or more past due, contractual outstanding principal | 70 us-gaap_FairValueOptionLoansHeldAsAssets90DaysOrMorePastDue / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_OtherLoansMember |
Loans receivable, fair value | 108 mbi_FairValueOptionAggregateFairValueLoansReceivable / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_OtherLoansMember | 711 mbi_FairValueOptionAggregateFairValueLoansReceivable / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_OtherLoansMember
Loans receivable, difference | 0 us-gaap_FairValueOptionAggregateDifferencesLoansAndLongTermReceivables / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_OtherLoansMember | 10 us-gaap_FairValueOptionAggregateDifferencesLoansAndLongTermReceivables / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_OtherLoansMember
Loans receivable, 90 days or more past due, difference | $ 70 us-gaap_FairValueOptionLoansHeldAsAssets90DaysOrMorePastDueAggregateDifference / dei_LegalEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember / us-gaap_ScheduleOfFairValueOfSeparateAccountsByMajorCategoryOfInvestmentAxis = mbi_OtherLoansMember |

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- Definition Fair Value Option Aggregate Differences Medium Term Notes + References No definition available. + Details
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- Definition Fair Value Option Aggregate Fair Value Loans Receivable + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Definition Fair Value Option Aggregate Fair Value Long Term Debt Instruments + References No definition available. + Details
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- Definition Fair Value Option Aggregate Fair Value Medium Term Notes + References No definition available. + Details
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- Definition Fair Value Option Contractual Outstanding Principal Loans Receivable + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Definition Fair Value Option Contractual Outstanding Principal Long Term Debt Instruments + References No definition available. + Details
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- Definition Fair Value Option Contractual Outstanding Principal Medium Term Notes + References No definition available. + Details
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- Definition This item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of loans and long-term receivables (other than securities categorized as trading, available-for-sale or held-to-maturity) that have contractual principal amounts and for which the fair value option has been elected. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (d)(1) -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14064-108612 + Details
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- Definition This item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of long-term debt instruments that have contractual principal amounts and for which the fair value option has been elected. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (d)(2) -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14064-108612 + Details
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- Definition This item represents the aggregate fair value of loans held as assets that are 90 days or more past due for which the fair value option has been elected. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (e)(1) -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14064-108612 + Details
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- Definition For loans held as assets for which the fair value option has been elected, this item represents the difference between the aggregate fair value and the aggregate unpaid principal balance of those loans that are 90 days or more past due. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (e)(3) -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14064-108612 + Details
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- Definition This item represents the aggregate fair value of loans held as assets that are in nonaccrual status for which the fair value option has been elected, if the entity's policy is to recognize interest income separately from other changes in fair value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 28 -Subparagraph (e)(2) -URI http://asc.fasb.org/extlink&oid=6957238&loc=d3e14064-108612 + Details
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