Loss and Loss Adjustment Expense Reserves (Schedule of Insurance Loss Recoverable) (Detail) - Insurance Loss Recoverable [Member] $ in Millions	9 Months Ended
Sep. 30, 2026 USD ($)
Roll forward of Insurance Loss Recoverable [Line Items]
Gross Reserve beginning balance, Insurance loss recoverable	$ 43
Collections for Cases	(10)
Accretion of Recoveries	1
Changes in Discount Rates	(3)
Changes in Assumptions	43
Gross Reserve ending balance, Insurance loss recoverable	$ 74

Cash receipts related to recoveries of previously paid claims

No definition available.

Carrying amount as of the balance sheet date of paid insurance policy claims and LAE expense expected to be recovered.

No definition available.

Name:	mbi_InsuranceLossRecoverable
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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Insurance Loss Recoverable And Changes In Recoveries Of Unpaid Losses Accretion Of Recoveries

No definition available.

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Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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Insurance Loss Recoverable And Changes In Recoveries Of Unpaid Losses Change In Assumptions

No definition available.

Name:	mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInAssumptions
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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Insurance Loss Recoverable And Changes In Recoveries Of Unpaid Losses Change In Discount Rate

No definition available.

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

Roll forward of Insurance Loss Recoverable [Line Items]

No definition available.

Name:	mbi_RollForwardOfInsuranceLossRecoverableLineItems
Namespace Prefix:	mbi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Name:	mbi_ClaimLiabilityAxis=mbi_InsuranceLossRecoverableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: