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Schedule IV - (Reinsurance) (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Supplemental Schedule of Reinsurance Premiums for Insurance Companies [Abstract] |  |  |
Direct Amount | $ (18)us-gaap_DirectPremiumsWritten | $ (40)us-gaap_DirectPremiumsWritten | $ 17 us-gaap_DirectPremiumsWritten
Ceded to Others | 2 us-gaap_CededPremiumsWritten | 2 us-gaap_CededPremiumsWritten | 2 us-gaap_CededPremiumsWritten
Assumed From Other Companies | 0 us-gaap_AssumedPremiumsWritten | 1 us-gaap_AssumedPremiumsWritten | 2 us-gaap_AssumedPremiumsWritten
Net Amount | $ (20)us-gaap_PremiumsWrittenNet | $ (41)us-gaap_PremiumsWrittenNet | $ 17 us-gaap_PremiumsWrittenNet
Percentage of Amount Assumed to Net | 0.00%mbi_PremiumsWrittenFinancialGuaranteeInsuranceContractsPercentageOfAmountAssumedToNet | (2.00%)mbi_PremiumsWrittenFinancialGuaranteeInsuranceContractsPercentageOfAmountAssumedToNet | 12.00%mbi_PremiumsWrittenFinancialGuaranteeInsuranceContractsPercentageOfAmountAssumedToNet

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- Definition Premiums Written Financial Guarantee Insurance Contracts Percentage Of Amount Assumed To Net + References No definition available. + Details
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- Definition Amount of written premiums assumed from other entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b)(2),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition Amount of written premiums ceded to other entities. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(2) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Definition Amount, before premiums ceded to other entities and premiums assumed by the entity, of premiums written. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c)(1) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount, after premiums ceded to other entities and premiums assumed by the entity, of premiums written. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 17 -Article 12 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 605 -Section 50 -Paragraph 1 -Subparagraph (b),(c) -URI http://asc.fasb.org/extlink&oid=6486672&loc=d3e27261-158547 + Details
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- Details
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