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Investments (Credit Ratings Of Asset-Backed And Mortgage-Backed Securities And Corporate Obligations In Available-For-Sale Investments For Which Fair Value Was Less Than Amortized Cost) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Fair value	$ 2,071us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue	$ 3,457us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Gross unrealized losses	(147)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments	(229)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	195us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Gross unrealized losses	(33)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	442us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Gross unrealized losses	(11)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	719us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized losses	(92)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	1,356us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Gross unrealized losses	(136)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Moody's Aaa [Member] | Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	58us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAaaRatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Gross unrealized losses	0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAaaRatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Moody's Aaa [Member] | Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	412us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAaaRatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Gross unrealized losses	(7)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAaaRatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Moody's Aaa [Member] | Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	249us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAaaRatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized losses	0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAaaRatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Moody's Aaa [Member] | Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	719us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAaaRatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Gross unrealized losses	(7)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAaaRatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Moodys Aa 1 Rating [Member] | Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	55us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Gross unrealized losses	(2)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Moodys Aa 1 Rating [Member] | Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	2us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Gross unrealized losses	0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Moodys Aa 1 Rating [Member] | Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	201us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized losses	(2)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Moodys Aa 1 Rating [Member] | Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	258us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Gross unrealized losses	(4)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Moody's A1 [Member] | Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	9us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Gross unrealized losses	0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Moody's A1 [Member] | Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	1us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Gross unrealized losses	0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Moody's A1 [Member] | Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	203us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized losses	(3)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Moody's A1 [Member] | Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	213us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Gross unrealized losses	(3)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Moody's Baa 1 Rating [Member] | Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	4us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Gross unrealized losses	0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Moody's Baa 1 Rating [Member] | Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	3us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Gross unrealized losses	0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Moody's Baa 1 Rating [Member] | Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	40us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized losses	(1)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Moody's Baa 1 Rating [Member] | Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	47us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Gross unrealized losses	(1)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Below Investment Grade [Member] | Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	69us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_BelowInvestmentGradeMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Gross unrealized losses	(31)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = mbi_BelowInvestmentGradeMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Below Investment Grade [Member] | Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	9us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_BelowInvestmentGradeMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Gross unrealized losses	0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = mbi_BelowInvestmentGradeMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Below Investment Grade [Member] | Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	3us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_BelowInvestmentGradeMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized losses	0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = mbi_BelowInvestmentGradeMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Below Investment Grade [Member] | Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	81us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_BelowInvestmentGradeMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Gross unrealized losses	(31)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = mbi_BelowInvestmentGradeMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Not Rated [Member] | Asset-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	0us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_NotRatedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Gross unrealized losses	0mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = mbi_NotRatedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Not Rated [Member] | Mortgage-backed [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	15us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_NotRatedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Gross unrealized losses	(4)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = mbi_NotRatedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember
Not Rated [Member] | Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	23us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_NotRatedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Gross unrealized losses	(86)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = mbi_NotRatedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember
Not Rated [Member] | Total ABS MBS and Corporate Obligations [Member]
Schedule Of Available For Sale Securities [Line Items]
Fair value	38us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_NotRatedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember
Gross unrealized losses	$ (90)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_CreditRatingMoodysAxis = mbi_NotRatedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember

X

- Definition

Amount of accumulated pre-tax unrealized losses before deducting pre-tax unrealized gains on investments in available-for-sale securities impacting investments.

+ References

No definition available.

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- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

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