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Loss And Loss Adjustment Expense Reserves (Schedule Of Loss And Loss Adjustment Expenses Reserves) (Detail) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2015	Sep. 30, 2014
Loss And Loss Adjustment Expense Reserves [Line Items]
Changes in unearned premium revenue		$ 274	$ 303
Changes in LAE reserves		24	$ 124
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Gross loss and LAE reserve, beginning balance		506
Loss payments for cases with reserves		(42)
Accretion of claim liability discount		6
Changes in discount rates		(28)
Changes in assumptions		46
Changes in unearned premium revenue		13
Changes in LAE reserves		(9)
Other	[1]	(12)
Gross loss and LAE reserve, ending balance		$ 480
[1] Primarily changes in amount and timing of payments.

X

- Definition

Financial Guarantee Insurance Contracts Claim Liability Significant Components Of Change In Assumptions

+ References

No definition available.

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- Definition

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

+ References

No definition available.

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X

- Definition

Loss And Loss Adjustment Expense Reserves [Line Items]

+ References

No definition available.

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Namespace Prefix:	mbi_
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X

- Definition

Loss payments for cases with reserves

+ References

No definition available.

+ Details

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Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

This element represents the amount of the change in the claim liability related to financial guarantee insurance contracts arising from the accretion of the discount of the claim liability.

+ References

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



-Publisher FASB



-Name Accounting Standards Codification



-Topic 944



-SubTopic 40



-Section 50



-Paragraph 9



-Subparagraph (a)(2)(ii)



-URI http://asc.fasb.org/extlink&oid=6933314&loc=SL5749324-161292

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Namespace Prefix:	us-gaap_
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Balance Type:	credit
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X

- Definition

This element represents the amount of change in the claim liability related to financial guarantee insurance contracts arising from a change in the rate used to discount the claim liability.

+ References

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



-Publisher FASB



-Name Accounting Standards Codification



-Topic 944



-SubTopic 40



-Section 50



-Paragraph 9



-Subparagraph (a)(2)(i)



-URI http://asc.fasb.org/extlink&oid=6933314&loc=SL5749324-161292

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X

- Definition

The increase (decrease) during the reporting period in the reserve account established to account for expected but unspecified losses.

+ References

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



-Publisher FASB



-Name Accounting Standards Codification



-Topic 230



-SubTopic 10



-Section 45



-Paragraph 28



-Subparagraph (a)



-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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- Definition

Amount of increase (decrease) unearned premiums written. Excludes portion of unearned premiums amortized into income.

+ References

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



-Publisher FASB



-Name Accounting Standards Codification



-Topic 230



-SubTopic 10



-Section 45



-Paragraph 28



-Subparagraph (a)



-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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X

- Definition

The amount needed to reflect the estimated ultimate cost of settling claims relating to insured events that have occurred on or before the balance sheet date, whether or not reported to the insurer at that date.

+ References

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



-Publisher FASB



-Name Accounting Standards Codification



-Topic 944



-SubTopic 40



-Section 50



-Paragraph 3



-Subparagraph (a),(b),(c)



-URI http://asc.fasb.org/extlink&oid=6482485&loc=d3e14764-158437

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