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

Shares issued under Omnibus Plan in connection with other than stock options, count.

+ References

No definition available.

+ Details

Name:	mbi_SharesIssuedUnderOtherThanStockOptionsPlanCountedInLongTermIncentivePlan
Namespace Prefix:	mbi_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Shares issued under stock options plan counted in long-term incentive plan.

+ References

No definition available.

+ Details

Name:	mbi_SharesIssuedUnderStockOptionsPlanCountedInLongTermIncentivePlan
Namespace Prefix:	mbi_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5047-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_AllocatedShareBasedCompensationExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Aggregate number of common shares reserved for future issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.29)
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

+ Details

Name:	us-gaap_CommonStockCapitalSharesReservedForFutureIssuance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- 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 not yet recognized for nonvested award under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Weighted-average period over which cost not yet recognized is expected to be recognized for award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of cash outflow to satisfy grantee's tax withholding obligation for award under share-based payment arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3291-108585

+ Details

Name:	us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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

- Definition

Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
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_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Total number of shares issued during the period, including shares forfeited, as a result of Restricted Stock Awards.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Aggregate value of stock related to Restricted Stock Awards issued during the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(28))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(29))
-URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

+ Details

Name:	us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

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

- Details

Name:	us-gaap_AwardTypeAxis=mbi_OmnibusPlanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=mbi_AfterAmendmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=mbi_PriorToAmendmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=mbi_StockPriceTargetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

Benefit Plans (Narrative) (Detail) - USD ($) $ in Millions	12 Months Ended
Benefit Plans (Narrative) (Detail) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Pension and 401k expense	$ 4	$ 4	$ 3
Defined contribution pension [Member]
Share-based Compensation Arrangement by Share-based Payment Award [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%
Qualified profit-sharing/401(k) [Member]
Share-based Compensation Arrangement by Share-based Payment Award [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%
Contributions percentage of employer match	5.00%
Maximum [Member] \| Defined contribution pension [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Maximum annual compensation subject to pension	$ 2
Omnibus Plan [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shareholder approved increase in common stock reserved and available for issuance	16,500,000
Shares issued under Omnibus Plan in connection with stock options, count	1
Shares/units available for future grants	3,338,822
Omnibus Plan [Member] \| After May 1, 2012 Amendment [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued under Omnibus Plan in connection with other than stock options, count	1.28
Omnibus Plan [Member] \| Prior To May 1, 2012 Amendment [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares issued under Omnibus Plan in connection with other than stock options, count	2
Restricted Stock [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock issued during period, value, restricted stock award, gross	$ 7	8
Share-based compensation expense (benefit)	11	12	6
Tax charge related to share-based compensation	0	$ 0	$ 0
Unrecognized compensation cost	$ 26
Expected to be recognized as expense over a weighted average period, years	2 years 5 months 12 days
Restricted Stock [Member] \| Stock Price Targets [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Restricted shares granted	502,738	221,213	247,286
Restricted Stock [Member] \| Minimum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Long-term incentive awards vesting period, years	3 years
Restricted Stock [Member] \| Maximum [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Long-term incentive awards vesting period, years	7 years