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Loss And Loss Adjustment Expense Reserves (Schedule Of Loss And Loss Adjustment Expenses Reserves) (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]
Changes in unearned premium revenue	$ 412us-gaap_IncreaseDecreaseInUnearnedPremiums		$ 506us-gaap_IncreaseDecreaseInUnearnedPremiums		$ 600us-gaap_IncreaseDecreaseInUnearnedPremiums
Changes in LAE reserves	133us-gaap_IncreaseDecreaseInLossAndLossAdjustmentExpenseReserve		216us-gaap_IncreaseDecreaseInLossAndLossAdjustmentExpenseReserve		(17)us-gaap_IncreaseDecreaseInLossAndLossAdjustmentExpenseReserve
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Gross loss andLAE reserve, beginning balance	641us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember		853us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember
Loss payments for cases with reserves	(157)mbi_LossPaymentsForCasesWithReserves / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember		(337)mbi_LossPaymentsForCasesWithReserves / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember
Accretion of claim liability discount	14us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeAccretionOfDiscount / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember		13us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeAccretionOfDiscount / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember
Changes in discount rates	79us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeEffectOfChangeInDiscountRate / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember		(92)us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeEffectOfChangeInDiscountRate / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember
Changes in assumptions	(35)mbi_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeInAssumptions / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember		141mbi_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeInAssumptions / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember
Changes in unearned premium revenue	14us-gaap_IncreaseDecreaseInUnearnedPremiums / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember		12us-gaap_IncreaseDecreaseInUnearnedPremiums / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember
Changes in LAE reserves	(19)us-gaap_IncreaseDecreaseInLossAndLossAdjustmentExpenseReserve / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember		(2)us-gaap_IncreaseDecreaseInLossAndLossAdjustmentExpenseReserve / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember
Other	(31)mbi_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeOther / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember	[1]	53mbi_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeOther / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember	[1]
Gross loss andLAE reserve, ending balance	$ 506us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember		$ 641us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember
[1] Primarily changes in amount and timing of payments.

X

- Definition

Financial Guarantee Insurance Contracts Claim Liability Significant Components Of Change In Assumptions

+ References

No definition available.

+ Details

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

X

- Definition

Changes in the timing and amount of net payments on financial guarantee contracts.

+ References

No definition available.

+ Details

Name:	mbi_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeOther
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Details

Name:	mbi_LossAndLossAdjustmentExpenseReservesLineItems
Namespace Prefix:	mbi_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Loss payments for cases with reserves

+ References

No definition available.

+ Details

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

X

- Definition

This element represents the amount of the change in the claim liability related to financial guarantee insurance contracts arising from the accretion of the discount of the claim liability.

+ 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)(2)(ii)

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

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeAccretionOfDiscount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

This element represents the amount of change in the claim liability related to financial guarantee insurance contracts arising from a change in the rate used to discount the claim liability.

+ 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)(2)(i)

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

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeEffectOfChangeInDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The increase (decrease) during the reporting period in the reserve account established to account for expected but unspecified losses.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

+ Details

Name:	us-gaap_IncreaseDecreaseInLossAndLossAdjustmentExpenseReserve
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Amount of increase (decrease) unearned premiums written. Excludes portion of unearned premiums amortized into income.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

+ Details

Name:	us-gaap_IncreaseDecreaseInUnearnedPremiums
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 40

-Section 50

-Paragraph 3

-Subparagraph (a),(b),(c)

-URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437

+ Details

Name:	us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	mbi_ClaimLiabilityAxis=mbi_LossAndLaeReservesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: