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Business Developments, Risks And Uncertainties, And Liquidity (Change In Credit Spreads (Asset/Liability Products Business) (Detail) (USD $) In Millions, unless otherwise specified | Sep. 30, 2012
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200 Basis Point Decrease [Member] |
Business Acquisition [Line Items] |
Estimated change in fair value | $ 149
50 Basis Point Decrease [Member] |
Business Acquisition [Line Items] |
Estimated change in fair value | 41
50 Basis Point Increase [Member] |
Business Acquisition [Line Items] |
Estimated change in fair value | (37)
200 Basis Point Increase [Member] |
Business Acquisition [Line Items] |
Estimated change in fair value | $ (130)

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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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