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Investments (Average Credit Ratings For Securities Rated Below Investment Grade Without The Effect Of Financial Guarantees) (Detail) (USD $) In Millions, unless otherwise specified | Sep. 30, 2013
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Asset-backed [Member] |
Schedule Of Investments [Line Items] |
Fair Value Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors | $ 101
Percentage Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors | 64.00%
Mortgage-backed [Member] |
Schedule Of Investments [Line Items] |
Fair Value Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors | 20
Percentage Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors | 99.00%
Corporate Obligations [Member] |
Schedule Of Investments [Line Items] |
Fair Value Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors | $ 0
Percentage Of Securities Rated Below Investment Grade Without Giving Effect To Benefit Of Guarantees Provided By Financial Guarantors | 0.00%

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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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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
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Namespace Prefix: | mbi_
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Period Type: | instant

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- Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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