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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
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |  |
Changes in unearned premium revenue | $ 412 us-gaap_IncreaseDecreaseInUnearnedPremiums |  | $ 506 us-gaap_IncreaseDecreaseInUnearnedPremiums |  | $ 600 us-gaap_IncreaseDecreaseInUnearnedPremiums
Changes in LAE reserves | 133 us-gaap_IncreaseDecreaseInLossAndLossAdjustmentExpenseReserve |  | 216 us-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 | 641 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  | 853 us-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 | 14 us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeAccretionOfDiscount / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  | 13 us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeAccretionOfDiscount / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  |
Changes in discount rates | 79 us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeEffectOfChangeInDiscountRate / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  | (92)us-gaap_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeEffectOfChangeInDiscountRate / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  |
Changes in assumptions | (35)mbi_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeInAssumptions / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  | 141 mbi_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeInAssumptions / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  |
Changes in unearned premium revenue | 14 us-gaap_IncreaseDecreaseInUnearnedPremiums / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  | 12 us-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] | 53 mbi_FinancialGuaranteeInsuranceContractsClaimLiabilitySignificantComponentsOfChangeOther / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember | [1] |
Gross loss andLAE reserve, ending balance | $ 506 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  | $ 641 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember |  |
[1] | Primarily changes in amount and timing of payments.

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- 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

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- 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

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

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- 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

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- 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

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

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

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- 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

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- 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

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