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Loss And Loss Adjustment Expense Reserves (Schedule Of Expenses Related To Remedial Actions For Insured Obligations) (Detail) (Remedial Actions For Insured Obligations [Member], USD $) In Millions, unless otherwise specified | 9 Months Ended
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Sep. 30, 2013 | Sep. 30, 2012
Remedial Actions For Insured Obligations [Member] |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |
Loss adjustment expense incurred, gross | $ 31 | $ 113

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- Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:stringItemType
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Period Type: | duration

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- Definition Amount, before effect of policies assumed or ceded, of expense related to provision for policy benefits and costs incurred. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-04.5) -URI http://asc.fasb.org/extlink&oid=6879464&loc=d3e573970-122913 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 04 -Paragraph 5 -Article 7 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration