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Pension And Profit Sharing Plans (Narrative) (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Defined Benefit Plan Disclosure [Line Items] |  |  |
Maximum annual compensation subject to pension | $ 1.5 mbi_AnnualCompensationMaximumPensionPerEmployeeAmount |  |
Defined contribution pension [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contribution percentage of eligible employees compensation | 10.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = us-gaap_DefinedContributionPensionMember |  |
Vested after two years | 20.00%mbi_PercentageOfPensionBenefitsVestedDueAfterTwoYears / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = us-gaap_DefinedContributionPensionMember |  |
Vested after three years | 60.00%mbi_PercentageOfPensionBenefitsVestedDueAfterThreeYears / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = us-gaap_DefinedContributionPensionMember |  |
Vested after four years | 80.00%mbi_PercentageOfPensionBenefitsVestedDueAfterFourYears / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = us-gaap_DefinedContributionPensionMember |  |
Vested after five years | 100.00%mbi_PercentageOfPensionBenefitsVestedDueAfterFiveYears / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = us-gaap_DefinedContributionPensionMember |  |
Pension expense | 4 us-gaap_PensionExpense / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = us-gaap_DefinedContributionPensionMember | 4 us-gaap_PensionExpense / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = us-gaap_DefinedContributionPensionMember | 7 us-gaap_PensionExpense / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = us-gaap_DefinedContributionPensionMember
Qualified profit-sharing/401(k) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contribution percentage of eligible employees compensation | 25.00%us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_QualifiedProfitSharingMember |  |
Vested after two years | 20.00%mbi_PercentageOfPensionBenefitsVestedDueAfterTwoYears / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_QualifiedProfitSharingMember |  |
Vested after three years | 60.00%mbi_PercentageOfPensionBenefitsVestedDueAfterThreeYears / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_QualifiedProfitSharingMember |  |
Vested after four years | 80.00%mbi_PercentageOfPensionBenefitsVestedDueAfterFourYears / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_QualifiedProfitSharingMember |  |
Vested after five years | 100.00%mbi_PercentageOfPensionBenefitsVestedDueAfterFiveYears / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_QualifiedProfitSharingMember |  |
Pension expense | 2 us-gaap_PensionExpense / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_QualifiedProfitSharingMember | 3 us-gaap_PensionExpense / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_QualifiedProfitSharingMember | 3 us-gaap_PensionExpense / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_QualifiedProfitSharingMember
Contributions percentage of employer match | 5.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_QualifiedProfitSharingMember |  |
Non-qualified pension [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension expense | 2 us-gaap_PensionExpense / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_NonqualifiedPensionMember | 2 us-gaap_PensionExpense / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_NonqualifiedPensionMember | 2 us-gaap_PensionExpense / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_NonqualifiedPensionMember
Non-qualified profit-sharing/401(k) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension expense | $ 1 us-gaap_PensionExpense / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_NonqualifiedProfitSharingMember | $ 1 us-gaap_PensionExpense / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_NonqualifiedProfitSharingMember | $ 1 us-gaap_PensionExpense / us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis = mbi_NonqualifiedProfitSharingMember

X
- Definition Maximum annual compensation per employee subject to pension contribution. + References No definition available. + Details
Name: | mbi_AnnualCompensationMaximumPensionPerEmployeeAmount
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Percentage of pension benefits vested due after five years. + References No definition available. + Details
Name: | mbi_PercentageOfPensionBenefitsVestedDueAfterFiveYears
Namespace Prefix: | mbi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of pension benefits vested due after four years. + References No definition available. + Details
Name: | mbi_PercentageOfPensionBenefitsVestedDueAfterFourYears
Namespace Prefix: | mbi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of pension benefits vested due after three years. + References No definition available. + Details
Name: | mbi_PercentageOfPensionBenefitsVestedDueAfterThreeYears
Namespace Prefix: | mbi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of pension benefits vested due after two years. + References No definition available. + Details
Name: | mbi_PercentageOfPensionBenefitsVestedDueAfterTwoYears
Namespace Prefix: | mbi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_DefinedBenefitPlanDisclosureLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanEmployerMatchingContributionPercentOfMatch
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
Name: | us-gaap_DefinedContributionPlanMaximumAnnualContributionsPerEmployeePercent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Definition The amount of pension benefit costs recognized during the period for (1) defined benefit plans and (2) defined contribution plans. For defined benefit plans, pension expense includes the following components: service cost, interest cost, expected return on plan assets, gain (loss) on plan assets, prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments. For defined contribution plans, the pension expense generally equals the firm's contribution to employees' accounts (if the firm contributes) during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=28361610&loc=d3e1928-114920 + Details
Name: | us-gaap_PensionExpense
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Details
Name: | us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis=mbi_NonqualifiedPensionMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis=mbi_NonqualifiedProfitSharingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis=mbi_QualifiedProfitSharingMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |