Aggregate CDO loss and LAE reserves.

No definition available.

Name:	mbi_AggregateCdoLossAndLaeReserves
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Elimination Of Loss And LAE Expense As Result Of Consolidating VIEs

+ References

No definition available.

+ Details

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

- Definition

Elimination Of Loss And Lae Reserves As Result Of Consolidating Vie

+ References

No definition available.

+ Details

Name:	mbi_Eliminationoflossandlaereservesasresultofconsolidatingvie
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Gross reserve, recoveries on unpaid losses

+ References

No definition available.

+ Details

Name:	mbi_GrossReserveRecoveriesOnUnpaidLosses
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of paid insurance policy claims and LAE expense expected to be recovered.

+ References

No definition available.

+ Details

Name:	mbi_InsuranceLossRecoverable
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Definition

Reserves For Expected Future Claim Payments

+ References

No definition available.

+ Details

Name:	mbi_ReservesForExpectedFutureClaimPayments
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Total Estimated Insurance Recoveries

+ References

No definition available.

+ Details

Name:	mbi_TotalEstimatedInsuranceRecoveries
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount as of the balance sheet date of amounts expected to be recovered under the terms of insurance contracts.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.17)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 17

-Article 5

+ Details

Name:	us-gaap_EstimatedInsuranceRecoveries
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This element represents the weighted average risk free rate used to discount the claim liability related to financial guarantee insurance contracts.

+ 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)(1)

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

+ Details

Name:	us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilityWeightedAverageRiskFreeDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after effects of policies assumed or ceded, of expense related to the provision for policy benefits and costs incurred.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.7-04.5)

-URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 5

-Article 7

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 944

-SubTopic 20

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	mbi_ClaimLiabilityAxis=us-gaap_FinancialGuaranteeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	mbi_ClassifiedStatusAxis=mbi_CdoPortfolioMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=mbi_FirstLienResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Loss And Loss Adjustment Expense Reserves [Line Items]
Policyholder (Benefits) And Claims Incurred Net	$ (6)	$ 50
Weighted average risk-free rate used to discount claim liability	1.49%
Insurance loss recoverable	527mbi_InsuranceLossRecoverable		533mbi_InsuranceLossRecoverable
CDO Portfolio [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Policyholder (Benefits) And Claims Incurred Net	2
CDO Portfolio [Member] \| Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Elimination Of Loss And LAE Expense (Benefit) As Result Of Consolidating VIEs	8
Second Lien Residential Mortgage Backed Securities And Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Policyholder (Benefits) And Claims Incurred Net	(8)	25
First Lien Residential Mortgage Backed Securities [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Policyholder (Benefits) And Claims Incurred Net	9	30
Financial Guarantee [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Aggregate CDO Loss And LAE Reserves	113		111
Financial Guarantee [Member] \| CDO Portfolio [Member] \| Variable Interest Entity Primary Beneficiary [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Elimination Of Loss And Lae Reserves As Result Of Consolidating VIEs	233		225
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Reserves for expected future claim payments	609		653
Expected recoveries of future payments	110		147
Recoveries On Unpaid Losses [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Expected recoveries of future payments	110		147
Insurance Loss Recoverable [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance loss recoverable	527
Insurance Loss Recoverable [Member] \| Classified [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance claim loss recoverable	527		533
Insurance loss recoverable	527		533
Loan Repurchase Commitments [Member] \| Second Lien Residential Mortgage Backed Securities And Reserves [Member] \| Ineligible Loan Recoveries [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Total estimated insurance recoveries	$ 385		$ 379