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Loss And Loss Adjustment Expense Reserves (RMBS Recoveries) (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Loss And Loss Adjustment Expense Reserves [Line Items]
Federal statutory rate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate	35.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Remaining weighted average contract period (in years)	7 years 8 months 12 days	9 years 8 months 12 days
Second-Lien RMBS Ineligible Mortgage Loans [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Contract claims on ineligible mortgage loans inception to date, incurred loss on transactions	417mbi_ContractClaimsOnIneligibleMortgageLoansInceptionToDateIncurredLossOnTransactions / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienRmbsIneligibleMortgageLoansMember

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- Definition

Contract Claims On Ineligible Mortgage Loans Inception To Date Incurred Loss On Transactions

+ References

No definition available.

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- Definition

Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 12

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32687-109319



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-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 6.I)

-URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817



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-Publisher FASB

-Name Accounting Standards Codification

-Topic 740

-SubTopic 10

-Section 50

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32698-109319

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Remaining weighted average contract period of financial guarantee insurance contracts, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 50

-Paragraph 9

-Subparagraph (a)(5)(ii)

-URI http://asc.fasb.org/extlink&oid=6933314&loc=SL5749324-161292



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-Name Accounting Standards Codification

-Topic 944

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-Section 55

-Paragraph 32

-URI http://asc.fasb.org/extlink&oid=6933462&loc=SL5751133-161288

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