Fair Value Of Financial Instruments (Narrative) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives, value under Direct Price Model and Dual-Default Model	7.00%
Net insured derivative liability	$ 207		$ 244
Impact of including the nonperformance risk in valuation of pre-tax net insured derivative liability	89		92
Percentage of level 3 assets at fair value in total assets measured at fair value value	20.00%		20.00%
Percentage of level 3 liabilities at fair value in total liabilities measured at fair value	53.00%		35.00%
Transfers into Level 3	0mbi_TransfersIntoLevelThree	160mbi_TransfersIntoLevelThree
Transfers out of Level 3	11mbi_TransfersOutOfLevelThree	14mbi_TransfersOutOfLevelThree
Transfers into Level 2	11mbi_TransfersIntoLevelTwo	14mbi_TransfersIntoLevelTwo
Transfers out of Level 2	0mbi_TransfersOutOfLevelTwo	160mbi_TransfersOutOfLevelTwo
Transfers into or out of Level 1	$ 0mbi_TransfersIntoAndOutOfLevelOne	$ 0mbi_TransfersIntoAndOutOfLevelOne
B E T Valuation Model [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Percentage of insured credit derivatives, fair value using the BET model	93.00%

Approximate percentage of level 3 assets at fair value in total assets measured at fair value.

No definition available.

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- Definition

Approximate percentage of level 3 liabilities at fair value in total liabilities measured at fair value.

+ References

No definition available.

+ Details

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- Definition

Impact Of Non Performance Risk On Insured Derivative Liability

+ References

No definition available.

+ Details

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- Definition

Percentage Of Insured Credit Derivatives Value Under Bet Model

+ References

No definition available.

+ Details

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- Definition

Percentage of insured credit derivatives value under direct price model and dual default model

+ References

No definition available.

+ Details

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- Definition

Transfers of assets measured at fair value on a recurring basis into and out of level one during the period.

+ References

No definition available.

+ Details

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- Definition

Transfers of assets measured at fair value on a recurring basis into level three during the period.

+ References

No definition available.

+ Details

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- Definition

Transfers of assets measured at fair value on a recurring basis into level two during the period.

+ References

No definition available.

+ Details

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- Definition

Transfers of assets measured at fair value on a recurring basis out of level three during the period.

+ References

No definition available.

+ Details

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- Definition

Transfers of assets measured at fair value on a recurring basis out of level two during the period.

+ References

No definition available.

+ Details

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- Definition

Fair values as of the balance sheet date of the net amount of all assets and liabilities resulting from contracts that meet the criteria of being accounted for as derivative instruments.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 45

-Paragraph 5

-URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958

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