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Investments (Average Credit Ratings And Fair Value and Percentage Of Guaranteed ABS MBS And Corporate Obligations Below Investment Grade) (Detail) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012
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Asset-Backed Securities [Member] |
Schedule of Available-for-sale Securities [Line Items] |
Fair Value | $ 101
Percentage | 65.00%
Mortgage-Backed Securities [Member] |
Schedule of Available-for-sale Securities [Line Items] |
Fair Value | $ 113
Percentage | 96.00%

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- Definition Fair Value of Securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors. + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Percentage of securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors. + References No definition available. + Details
Name: | mbi_PercentageOfSecuritiesRatedBelowInvestmentGradeWithoutGivingEffectToBenefitOfGuaranteesProvidedByFinancialGuarantors
Namespace Prefix: | mbi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

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- Details
Name: | us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration