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

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Changes in the timing and amount of net collections on financial guarantee contracts.

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