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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 | Mar. 31, 2015 | Dec. 31, 2014
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Schedule Of Available For Sale Securities [Line Items] |  |
Fair value | $ 1,360 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue | $ 2,071 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Asset-backed [Member] |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Fair value | 156 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Gross unrealized losses | (34)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Mortgage-backed [Member] |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Fair value | 388 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Gross unrealized losses | (8)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Corporate Obligations [Member] |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Fair value | 457 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Gross unrealized losses | (103)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 | 1,001 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember |
Gross unrealized losses | (145)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember |
Moody's Aaa [Member] | Asset-backed [Member] |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Fair value | 42 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAaaRatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Gross unrealized losses | 0 mbi_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 | 358 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAaaRatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Gross unrealized losses | (4)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 | 174 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAaaRatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Gross unrealized losses | 0 mbi_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 | 574 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAaaRatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember |
Gross unrealized losses | (4)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 | 50 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Gross unrealized losses | (1)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 | 2 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Gross unrealized losses | 0 mbi_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 | 121 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Gross unrealized losses | (1)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 | 173 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysAa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember |
Gross unrealized losses | (2)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 | 3 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Gross unrealized losses | 0 mbi_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 | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Gross unrealized losses | 0 mbi_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 | 129 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Gross unrealized losses | (1)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 | 132 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysA1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember |
Gross unrealized losses | (1)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 | 3 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Gross unrealized losses | 0 mbi_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 | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = us-gaap_MoodysBaa1RatingMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Gross unrealized losses | 0 mbi_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 | 20 us-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 | 23 us-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 | 58 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_BelowInvestmentGradeMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Gross unrealized losses | (33)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 | 14 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_BelowInvestmentGradeMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_MortgageBackedSecuritiesMember |
Gross unrealized losses | 0 mbi_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 | 3 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_BelowInvestmentGradeMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Gross unrealized losses | 0 mbi_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 | 75 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_BelowInvestmentGradeMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember |
Gross unrealized losses | (33)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 | 0 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_NotRatedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember |
Gross unrealized losses | 0 mbi_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 | 14 us-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 | 10 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_NotRatedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateDebtSecuritiesMember |
Gross unrealized losses | (100)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 | 24 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_CreditRatingMoodysAxis = mbi_NotRatedMember / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = mbi_AbsMbsCorporateObligationsMember |
Gross unrealized losses | $ (104)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. + Details
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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 + Details
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