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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, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Total premiums earned	$ 122	$ 112	$ 308	$ 377
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	78	82	216	297
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	9	9	28	26
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	2	3	6	9
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	4	2	7	8
Central And South America [Member]
Segment Reporting Information [Line Items]
Total premiums earned	26	13	42	28
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	1	3	3
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 2	$ 2	$ 6	$ 6

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

- Details

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