Derivative Instruments (Narrative) (Detail) (USD $)	3 Months Ended	12 Months Ended
Mar. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Maximum amount of future guarantee payments	$ 8,300,000,000
Cash collateral posted to derivative counterparties	127,000,000	12,000,000
Securities posted as collateral to derivative counterparties	215,000,000	198,000,000
Number of credit support annexes	1mbi_NumberOfCreditSupportAnnexes	1mbi_NumberOfCreditSupportAnnexes
Fair value of Credit Support Annex	1,000,000mbi_FairValueOfCreditSupportAnnexes	2,000,000mbi_FairValueOfCreditSupportAnnexes
Amount of counterparty netting	1,000,000	81,000,000
Derivative assets, after counterparty netting	13,000,000	10,000,000
Other assets and other assets of consolidated VIEs, after counterparty netting and embedded reclass	6,000,000	2,000,000
Counterparty netting	1,000,000	81,000,000
Derivative liabilities, after counterparty netting and posting of cash collateral	481,000,000	456,000,000
Derivative liabilities and derivative liabilities of consolidated VIEs, after counterparty netting and embedded reclass	467,000,000	437,000,000
Derivative Liabilities [Member]
Derivative [Line Items]
Cash collateral posted to derivative counterparties	108,000,000	0
Other Liabilities [Member]
Derivative [Line Items]
Cash collateral posted to derivative counterparties	19,000,000	12,000,000
Other Assets [Member]
Derivative [Line Items]
Cash collateral posted to derivative counterparties		0
Medium Term Notes And Other Investments [Member] \| Embedded [Member]
Derivative [Line Items]
Embedded derivative assets	7,000,000	8,000,000
Total derivative liabilities	$ 14,000,000	$ 19,000,000

Derivative Asset Counterparty Netting Amount Fair Value

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- Definition

Derivative Fair Value Of Derivative Asset Net Of Counterparty Netting

+ References

No definition available.

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- Definition

Derivative Fair Value Of Derivative Liability Net Of Counterparty Netting

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No definition available.

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- Definition

Derivative Liability Counterparty Netting

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No definition available.

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- Definition

Fair value of credit support annexes which define the terms or rules under which collateral is posted or transferred between swap counterparties to mitigate the credit risk arising from "in the money" derivative positions.

+ References

No definition available.

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- Definition

Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees after (net of) reduction for potential recoveries under recourse or collateralization provisions.

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No definition available.

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Number of credit support annexes which define the terms or rules under which collateral is posted or transferred between swap counterparties to mitigate the credit risk arising from "in the money" derivative positions.

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No definition available.

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- Definition

Derivative Fair Value Of Derivative Assets Reported In Derivative Assets And Derivative Assets VIEs Net Of Counterparty Netting And Embedded Reclasses

+ References

No definition available.

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- Definition

Derivative Fair Value Of Derivative Liabilities Reported In Derivative Liabilities And Derivative Liavilities VIEs Net Of Counterparty Netting And Embedded Reclasses

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No definition available.

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- Definition

Amount of right to receive cash collateral under master netting arrangements that have not been offset against derivative liabilities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 13

-URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226016-175313

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (d)(2)

-URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 8

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=d3e41678-113959

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Amount of right to receive securities collateral under master netting arrangements that have not been offset against derivative liabilities.

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- Definition

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (a),(c)

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-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313

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Fair value, before effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities elected not to be offset. Excludes liabilities not subject to a master netting arrangement.

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