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Business Developments And Risks And Uncertainties (Assets And Liabilities Held For Sale) (Detail) (USD $) In Millions, unless otherwise specified | Dec. 31, 2014 | Dec. 31, 2013
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Assets And Liabilities Held For Sale By Major Category [Line Items] |  |
Assets held for sale | $ 802 us-gaap_OtherAssetsHeldForSaleCurrent | $ 29 us-gaap_OtherAssetsHeldForSaleCurrent
Liabilities held for sale | 772 us-gaap_LiabilitiesOfAssetsHeldForSale | 0 us-gaap_LiabilitiesOfAssetsHeldForSale
Investments At Fair Value [Member] |  |
Assets And Liabilities Held For Sale By Major Category [Line Items] |  |
Assets held for sale | 6 us-gaap_OtherAssetsHeldForSaleCurrent / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = mbi_InvestmentsAtFairValueMember | 0 us-gaap_OtherAssetsHeldForSaleCurrent / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = mbi_InvestmentsAtFairValueMember
Cash And Cash Equivalents [Member] |  |
Assets And Liabilities Held For Sale By Major Category [Line Items] |  |
Assets held for sale | 15 us-gaap_OtherAssetsHeldForSaleCurrent / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_CashAndCashEquivalentsMember | 0 us-gaap_OtherAssetsHeldForSaleCurrent / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_CashAndCashEquivalentsMember
Facility [Member] |  |
Assets And Liabilities Held For Sale By Major Category [Line Items] |  |
Assets held for sale | 26 us-gaap_OtherAssetsHeldForSaleCurrent / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = mbi_FacilityMember | 29 us-gaap_OtherAssetsHeldForSaleCurrent / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = mbi_FacilityMember
Other Assets [Member] |  |
Assets And Liabilities Held For Sale By Major Category [Line Items] |  |
Assets held for sale | 4 us-gaap_OtherAssetsHeldForSaleCurrent / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_OtherCurrentAssetsMember | 0 us-gaap_OtherAssetsHeldForSaleCurrent / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_OtherCurrentAssetsMember
Cash [Member] |  |
Assets And Liabilities Held For Sale By Major Category [Line Items] |  |
Assets held for sale | 40 us-gaap_OtherAssetsHeldForSaleCurrent / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_CashMember | 0 us-gaap_OtherAssetsHeldForSaleCurrent / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_CashMember
Loans Receivable [Member] |  |
Assets And Liabilities Held For Sale By Major Category [Line Items] |  |
Assets held for sale | 711 us-gaap_OtherAssetsHeldForSaleCurrent / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_LoansReceivableMember | 0 us-gaap_OtherAssetsHeldForSaleCurrent / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_LoansReceivableMember
Other Liabilities [Member] |  |
Assets And Liabilities Held For Sale By Major Category [Line Items] |  |
Liabilities held for sale | 18 us-gaap_LiabilitiesOfAssetsHeldForSale / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_OtherCurrentLiabilitiesMember | 0 us-gaap_LiabilitiesOfAssetsHeldForSale / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_OtherCurrentLiabilitiesMember
Variable Interest Entities Notes [Member] |  |
Assets And Liabilities Held For Sale By Major Category [Line Items] |  |
Liabilities held for sale | 431 us-gaap_LiabilitiesOfAssetsHeldForSale / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_NotesPayableOtherPayablesMember | 0 us-gaap_LiabilitiesOfAssetsHeldForSale / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_NotesPayableOtherPayablesMember
Payable For Loans Purchased [Member] |  |
Assets And Liabilities Held For Sale By Major Category [Line Items] |  |
Liabilities held for sale | $ 323 us-gaap_LiabilitiesOfAssetsHeldForSale / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_LoansPayableMember | $ 0 us-gaap_LiabilitiesOfAssetsHeldForSale / us-gaap_LongLivedAssetsHeldForSaleByAssetTypeAxis = us-gaap_LoansPayableMember

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- Definition Liability (such as a mortgage) related to a disposal group that is held for sale and anticipated to be sold in less than one year. The liability is expected to be discharged as part of the plan of sale for the asset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.15) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e1107-107759 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6360339&loc=d3e1436-107760 + Details
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- Definition The lower of the carrying value or fair value less cost to sell of current assets held for sale that are not elsewhere specified in the taxonomy. The carrying value of major classes of assets and liabilities of the disposal group are separately disclosed in the balance sheet in the specific concepts provided. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 205 -SubTopic 20 -Section 45 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=6892542&loc=d3e1107-107759 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 360 -SubTopic 10 -Section 35 -Paragraph 43 -URI http://asc.fasb.org/extlink&oid=29634951&loc=d3e1739-110224 + Details
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