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Loss And Loss Adjustment Expense Reserves (Schedule Of Present Value Of The Probability-Weighted Future Claim Payments And Recoveries) (Detail) (USD $) In Millions, unless otherwise specified | Mar. 31, 2015 | Dec. 31, 2014
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Loss And Loss Adjustment Expense Reserves [Line Items] |  |
LAE insurance loss recoverable | $ 0 mbi_LaeInsuranceLossRecoverable | $ 0 mbi_LaeInsuranceLossRecoverable
Insurance loss recoverable | (527)mbi_InsuranceLossRecoverable | (533)mbi_InsuranceLossRecoverable
Reinsurance recoverable on unpaid losses | 6 us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims | 6 us-gaap_ReinsuranceReceivablesIncurredButNotReportedClaims
Reinsurance recoverable on unpaid LAE | 1 mbi_ReinsuranceRecoverableOnUnpaidLossAdjustmentExpenses | 1 mbi_ReinsuranceRecoverableOnUnpaidLossAdjustmentExpenses
Reinsurance recoverable on paid and unpaid losses | 7 us-gaap_ReinsuranceReceivablesCaseBasis | 7 us-gaap_ReinsuranceReceivablesCaseBasis
Loss Reserves Claim Liability [Member] | Classified [Member] |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |
Loss and loss adjustment expense reserves | 464 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossReservesClaimLiabilityMember / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember | 464 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossReservesClaimLiabilityMember / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember
Lae Reserves [Member] | Classified [Member] |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |
Loss and loss adjustment expense reserves | 35 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LaeReservesMember / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember | 42 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LaeReservesMember / mbi_ClassifiedStatusAxis = mbi_ClassifiedMember
Loss And Lae Reserves [Member] |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |
Loss and loss adjustment expense reserves | $ 499 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember | $ 506 us-gaap_LiabilityForClaimsAndClaimsAdjustmentExpense / mbi_ClaimLiabilityAxis = mbi_LossAndLaeReservesMember

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- 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
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Carrying amount as of the balance sheet date of paid LAE expenses expected to be recovered. + References No definition available. + Details
Name: | mbi_LaeInsuranceLossRecoverable
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Details
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- Definition Reinsurance Recoverable On Unpaid LAE + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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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
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Period Type: | instant

X
- Definition The sum for all reinsurance arrangements of the known and estimated amount recoverable as of the balance sheet date from a given reinsurer for claims paid or incurred by the ceding insurer and associated claims settlement expenses for each reinsurance arrangement (or, the sum of all cases determined individually, probably applying different assumptions to each arrangement). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.6) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 + Details
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Balance Type: | debit
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- Definition Estimated amount after valuation allowance of reinsurance recoverables, due from reinsurers for incurred claims not yet reported, and claims settlement expenses. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21412-158489 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 310 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6485522&loc=d3e21409-158489 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 03 -Paragraph 6 -Article 7 + Details
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- Details
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- Details
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Balance Type: |
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- Details
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