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Insurance In Force (Schedule Of Insurance In Force By Type Of Bond, Excluding Transactions Guaranteed By MBIA Corp. On Behalf Of Various Investment Management Services And Other Affiliated Companies) (Parenthetical) (Detail) - USD ($) $ in Billions	Dec. 31, 2020	Dec. 31, 2019
Insurance in Force	$ 91.4	$ 107.5
Gross Par Amount	$ 49.6	58.9
Corporate Asset Backed [Member] | Structured Insurance Securitizations [Member]
Insurance in Force		2.1
Gross Par Amount		$ 1.0

X

- Definition

Represents the amount of insured contractual payments (principal and interest) outstanding on the underlying insured obligations for financial guaranty contracts, gross of ceded contracts.

+ References

No definition available.

+ Details

Name:	mbi_InsuredFinancialObligationsInsuredContractualPaymentsOutstandingGross
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Represents the outstanding principal amounts of the underlying insured obligations for financial guaranty contracts, gross of ceded contracts.

+ References

No definition available.

+ Details

Name:	mbi_InsuredFinancialObligationsOutstandingPrincipalAmountGross
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	us-gaap_UnderlyingAssetClassAxis=mbi_CorporateAssetBackedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=mbi_StructuredInsuranceSecuritizationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: