Maximum annual compensation per employee subject to pension contribution.

No definition available.

Name:	mbi_AnnualCompensationMaximumPensionPerEmployeeAmount
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- 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

- 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

- 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

- 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

- 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

- 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

- 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

- 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

- Details

Name:	us-gaap_DeferredCompensationArrangementWithIndividualPostretirementBenefitsByTypeOfDeferredCompensationAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

Pension And Profit Sharing Plans (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
	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