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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
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Schedule Of Available For Sale Securities [Line Items] |  |
Fair value | $ 2,071 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue | $ 3,457 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Gross unrealized losses | (147)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments | (229)mbi_AvailableForSaleSecuritiesGrossUnrealizedLossAccumulatedInInvestments
Asset-backed [Member] |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Fair value | 195 us-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 | 442 us-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 | 719 us-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 | 1,356 us-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 | 58 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 | 412 us-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 | 249 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 | 719 us-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 | 55 us-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 | 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 | 201 us-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 | 258 us-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 | 9 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 | 1 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 | 203 us-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 | 213 us-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 | 4 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 | 3 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 | 40 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 | 47 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 | 69 us-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 | 9 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 | 81 us-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 | 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 | 15 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 | 23 us-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 | 38 us-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. + 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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