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Loss And Loss Adjustment Expense Reserves (Schedule Of Insurance Loss Recoverable And Changes In Recoveries On Unpaid Losses) (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015
Loss And Loss Adjustment Expense Recoveries [Line Items] |  |
Gross Reserve beginning balance, Insurance loss recoverable | $ 533 mbi_InsuranceLossRecoverable |
Gross Reserve beginning balance, Total | 680 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLosses |
Collections for Cases with Recoveries | (21)mbi_CollectionsForCasesWithRecoveries |
Accretion of Recoveries | 3 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesAccretionOfRecoveries |
Changes in Discount Rates | 8 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInDiscountRate |
Changes in Assumptions | (26)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInAssumptions |
Changes in LAE Recoveries | (1)mbi_IncreaseDecreaseInLossAdjustmentExpenseRecoveries |
Other | (6)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesOther | [1]
Gross Reserve ending balance, Insurance loss recoverable | 527 mbi_InsuranceLossRecoverable |
Gross Reserve ending balance, Total | 637 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLosses |
Insurance Loss Recoverable [Member] |  |
Loss And Loss Adjustment Expense Recoveries [Line Items] |  |
Collections for Cases with Recoveries | (21)mbi_CollectionsForCasesWithRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |
Accretion of Recoveries | 2 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesAccretionOfRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |
Changes in Discount Rates | 5 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInDiscountRate / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |
Changes in Assumptions | 14 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInAssumptions / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |
Changes in LAE Recoveries | 0 mbi_IncreaseDecreaseInLossAdjustmentExpenseRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |
Other | (6)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesOther / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember | [1]
Gross Reserve ending balance, Insurance loss recoverable | 527 mbi_InsuranceLossRecoverable / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |
Recoveries On Unpaid Losses [Member] |  |
Loss And Loss Adjustment Expense Recoveries [Line Items] |  |
Gross Reserve beginning balance,Recoveries on unpaid losses | 147 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
Collections for Cases with Recoveries | 0 mbi_CollectionsForCasesWithRecoveries / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
Accretion of Recoveries | 1 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesAccretionOfRecoveries / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
Changes in Discount Rates | 3 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInDiscountRate / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
Changes in Assumptions | (40)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInAssumptions / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
Changes in LAE Recoveries | (1)mbi_IncreaseDecreaseInLossAdjustmentExpenseRecoveries / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
Other | 0 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesOther / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember | [1]
Gross Reserve ending balance, Recoveries on unpaid losses | $ 110 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
[1] | Primarily changes in amount and timing of collections.

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- Definition Cash receipts related to recoveries of previously paid claims + References No definition available. + Details
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Period Type: | duration

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- Definition Gross reserve, recoveries on unpaid losses + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Increase Decrease In Loss Adjustment Expense Recoveries + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
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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
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- Definition Insurance loss recoverable and changes in recoveries of unpaid losses. + References No definition available. + Details
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- Definition Insurance Loss Recoverable And Changes In Recoveries Of Unpaid Losses Accretion Of Recoveries + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
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- Definition Insurance Loss Recoverable And Changes In Recoveries Of Unpaid Losses Change In Assumptions + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
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- Definition Insurance Loss Recoverable And Changes In Recoveries Of Unpaid Losses Change In Discount Rate + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition Changes in the timing and amount of net collections on financial guarantee contracts. + References No definition available. + Details
Name: | mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesOther
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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- Details
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Namespace Prefix: |
Data Type: | na
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Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |