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Investments (Securities Held In Unrealized Loss Position And Insured By Financial Guarantor) (Detail) - Mortgage-backed [Member] - Financial Guarantee [Member] $ in Millions | Dec. 31, 2021 USD ($)
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Schedule Of Investments [Line Items] |  |
Total Available-For-Sale, Fair Value | $ 1 |
Gross unrealized losses | 0 |
Loss and loss adjustment expense reserves | $ 0 | [1]
[1] | Insurance loss reserve estimates are based on the proportion of par value owned to the total amount of par value insured and are discounted using a discount rate equal to the risk-free rate applicable to the currency and weighted average remaining life of the insurance contract and may differ from the fair value.

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- Definition Amount of accumulated pre-tax unrealized losses before deducting pre-tax unrealized gains on investments in available-for-sale securities impacting investments. + References No definition available. + Details
Name: | mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events by financial guarantor that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date. + References No definition available. + Details
Name: | mbi_LiabilityForClaimsAndClaimsAdjustmentExpenseInsuredByFinancialGuarantor
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
Name: | us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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_ScheduleOfInvestmentsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_GuaranteeObligationsByNatureAxis=us-gaap_FinancialGuaranteeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |