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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 | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013
Loss And Loss Adjustment Expense Recoveries [Line Items] |  |  |  |
Gross Reserve beginning balance, Total | $ 900 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLosses |  | $ 3,980 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLosses |
Collections for Cases with Recoveries | (108)mbi_CollectionsForCasesWithRecoveries |  | (3,011)mbi_CollectionsForCasesWithRecoveries |
Accretion of Recoveries | 17 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesAccretionOfRecoveries |  | 25 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesAccretionOfRecoveries |
Changes in Discount Rates | 33 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInDiscountRate |  | (71)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInDiscountRate |
Changes in Assumptions | (156)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInAssumptions |  | 35 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInAssumptions |
Changes in LAE Recoveries | 1 mbi_IncreaseDecreaseInLossAdjustmentExpenseRecoveries |  | (41)mbi_IncreaseDecreaseInLossAdjustmentExpenseRecoveries |
Other | (7)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesOther | [1] | (17)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesOther | [1]
Gross Reserve ending balance, Total | 680 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLosses |  | 900 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLosses |
Insurance Loss Recoverable [Member] |  |  |  |
Loss And Loss Adjustment Expense Recoveries [Line Items] |  |  |  |
Gross Reserve beginning balance, Insurance loss recoverable | 694 mbi_InsuranceLossRecoverable / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |  | 3,648 mbi_InsuranceLossRecoverable / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |
Collections for Cases with Recoveries | (108)mbi_CollectionsForCasesWithRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |  | (3,011)mbi_CollectionsForCasesWithRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |
Accretion of Recoveries | 13 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesAccretionOfRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |  | 19 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesAccretionOfRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |
Changes in Discount Rates | 18 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInDiscountRate / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |  | (33)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInDiscountRate / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |
Changes in Assumptions | (77)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInAssumptions / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |  | 126 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInAssumptions / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |
Changes in LAE Recoveries | 0 mbi_IncreaseDecreaseInLossAdjustmentExpenseRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |  | (38)mbi_IncreaseDecreaseInLossAdjustmentExpenseRecoveries / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |
Other | (7)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesOther / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember | [1] | (17)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesOther / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember | [1]
Gross Reserve ending balance, Insurance loss recoverable | 533 mbi_InsuranceLossRecoverable / mbi_ClaimLiabilityAxis = mbi_InsuranceLossRecoverableMember |  | 694 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 | 206 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |  | 332 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
Collections for Cases with Recoveries | 0 mbi_CollectionsForCasesWithRecoveries / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |  | 0 mbi_CollectionsForCasesWithRecoveries / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
Accretion of Recoveries | 4 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesAccretionOfRecoveries / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |  | 6 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesAccretionOfRecoveries / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
Changes in Discount Rates | 15 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInDiscountRate / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |  | (38)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInDiscountRate / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
Changes in Assumptions | (79)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInAssumptions / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |  | (91)mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesChangeInAssumptions / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
Changes in LAE Recoveries | 1 mbi_IncreaseDecreaseInLossAdjustmentExpenseRecoveries / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |  | (3)mbi_IncreaseDecreaseInLossAdjustmentExpenseRecoveries / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
Other | 0 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesOther / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember | [1] | 0 mbi_InsuranceLossRecoverableAndChangesInRecoveriesOfUnpaidLossesOther / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember | [1]
Gross Reserve ending balance, Recoveries on unpaid losses | $ 147 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |  | $ 206 mbi_GrossReserveRecoveriesOnUnpaidLosses / mbi_ClaimLiabilityAxis = mbi_RecoveriesOnUnpaidLossesMember |
[1] | Primarily changes in amount and timing of collections.

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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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- 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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- Definition Insurance Loss Recoverable And Changes In Recoveries Of Unpaid Losses Change In Assumptions + References No definition available. + Details
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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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- Definition Changes in the timing and amount of net collections on financial guarantee contracts. + References No definition available. + Details
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- Details
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- Details
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- Details
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