Commitments and Contingencies (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
Sep. 30, 2014 Lawsuits
Commitments and Contingencies [Abstract]
Other material lawsuits pending	0
Number of operating lease term extensions	2
Lease renewal term	5 years
Operating leases future minimum payments due	$ 42
Lease incentives	$ 6

Amount of lease incentives from lessor to fund certain leasehold improvements.

No definition available.

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- Definition

Number of times lessee can extend the term of the operating lease.

+ References

No definition available.

+ Details

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- Definition

Other material lawsuits pending

+ References

No definition available.

+ Details

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Namespace Prefix:	mbi_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Details

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- Definition

Term of the lessee's leasing arrangement renewal, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

Name:	us-gaap_LesseeLeasingArrangementsOperatingLeasesRenewalTerm
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
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- Definition

Amount of required minimum rental payments for leases having an initial or remaining non-cancelable letter-terms in excess of one year.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 10

-Section 55

-Paragraph 40

-Subparagraph (Note 3)

-URI http://asc.fasb.org/extlink&oid=6584154&loc=d3e38371-112697

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 840

-SubTopic 20

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6453985&loc=d3e41502-112717

+ Details

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Balance Type:	credit
Period Type:	instant