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Pension And Profit Sharing Plans (Narrative) (Detail) - USD ($) $ in Millions | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Defined contribution pension [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contribution percentage of eligible employees compensation | 10.00% |  |
Vested after two years | 20.00% |  |
Vested after three years | 60.00% |  |
Vested after four years | 80.00% |  |
Vested after five years | 100.00% |  |
Pension and 401k expense | $ 1.0 | $ 2.0 | $ 1.0
Defined contribution pension [Member] | Maximum [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Maximum annual compensation subject to pension | $ 2.0 |  |
Qualified profit-sharing/401(k) [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Contribution percentage of eligible employees compensation | 25.00% |  |
Vested after two years | 20.00% |  |
Vested after three years | 60.00% |  |
Vested after four years | 80.00% |  |
Vested after five years | 100.00% |  |
Pension and 401k expense | $ 1.0 | 1.0 | 1.0
Contributions percentage of employer match | 5.00% |  |
Non-qualified pension [Member] |  |  |
Defined Benefit Plan Disclosure [Line Items] |  |  |
Pension and 401k expense | $ 1.0 | $ 3.0 | $ 3.0

X
- Definition Maximum annual compensation per employee subject to pension contribution. + References No definition available. + Details
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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
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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
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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
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Namespace Prefix: | mbi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + 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 Amount of cost (reversal of cost) for pension benefits. Excludes other postretirement benefits. + References No definition available. + 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=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
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: |

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