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Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Total premiums earned	$ 36	$ 37	$ 53
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	29	29	45
Other Americas [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 7	$ 8	$ 8

X

- Definition

Premiums earned net financial guarantee insurance contracts and insured derivatives.

+ References

No definition available.

+ Details

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Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

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

X

- Details

Name:	srt_StatementGeographicalAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	srt_StatementGeographicalAxis=mbi_OtherAmericasMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: