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Loss And Loss Adjustment Expense Reserves (Schedule Of Losses And Loss Adjustment Expenses) (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended
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Mar. 31, 2015 | Mar. 31, 2014
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |
Policyholder Benefits And Claims Incurred Net | $ (6)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet |  | $ 50 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet |
U S Public Finance Insurance [Member] |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |
Policyholder Benefits And Claims Incurred Net | (6)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_USPublicFinanceInsuranceMember |  | (14)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_USPublicFinanceInsuranceMember |
Second-Lien RMBS [Member] |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |
Policyholder Benefits And Claims Incurred Net | (8)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember |  | 25 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_SecondLienResidentialMortgageBackedSecuritiesAndReservesMember |
First-Lien RMBS [Member] |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |
Policyholder Benefits And Claims Incurred Net | 9 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_FirstLienResidentialMortgageBackedSecuritiesMember |  | 30 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_FirstLienResidentialMortgageBackedSecuritiesMember |
Other [Member] |  |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |  |
Policyholder Benefits And Claims Incurred Net | $ (1)us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_OtherLossesAndLossAndLossAdjustmentExpenseMember | [1] | $ 9 us-gaap_PolicyholderBenefitsAndClaimsIncurredNet / us-gaap_StatementBusinessSegmentsAxis = mbi_OtherLossesAndLossAndLossAdjustmentExpenseMember | [1]
[1] | Includes ABS CDOs, CMBS, non-U.S. public finance and other issues.

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- Definition Amount, after effects of policies assumed or ceded, of expense related to the 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 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 20 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6480758&loc=d3e6811-158387 + Details
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