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Loss And Loss Adjustment Expense Reserves (Schedule Of Loss And Loss Adjustment Expenses Reserves) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Loss And Loss Adjustment Expense Reserves [Line Items]
Gross loss and LAE reserve, beginning balance	$ 979
Changes in unearned premium revenue	39	$ 45
Gross loss and LAE reserve, ending balance	1,006
Loss And Lae Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Gross loss and LAE reserve, beginning balance	979
Loss payments for cases with reserves	(81)
Accretion of claim liability discount	5
Changes in discount rates	11
Changes in assumptions	79
Changes in unearned premium revenue	2
Changes in LAE	8
Other	3
Gross loss and LAE reserve, ending balance	$ 1,006

X

- Definition

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

+ References

No definition available.

+ Details

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Namespace Prefix:	mbi_
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X

- Definition

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

+ References

No definition available.

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Increase Decrease In Liability For Claim Adjustment Expense Reserve

+ References

No definition available.

+ Details

Name:	mbi_IncreaseDecreaseInLiabilityForClaimAdjustmentExpenseReserve
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Loss And Loss Adjustment Expense Reserves [Line Items]

+ References

No definition available.

+ Details

Name:	mbi_LossAndLossAdjustmentExpenseLineItems
Namespace Prefix:	mbi_
Data Type:	xbrli:stringItemType
Balance Type:	na
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X

- Definition

Loss payments for cases with reserves

+ References

No definition available.

+ Details

Name:	mbi_LossPaymentsForCasesWithReserves
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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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=82853773&loc=SL5749324-161292

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Namespace Prefix:	us-gaap_
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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=82853773&loc=SL5749324-161292

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Balance Type:	debit
Period Type:	duration

X

- 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=98514028&loc=d3e3602-108585

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Balance Type:	debit
Period Type:	duration

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)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (cc)
-URI http://asc.fasb.org/extlink&oid=65670824&loc=d3e14764-158437

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Period Type:	instant

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