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Variable Interest Entities (Narrative) (Detail) (USD $) In Millions, unless otherwise specified | 3 Months Ended |
--- | --- | ---
Mar. 31, 2015 | Mar. 31, 2014 | Dec. 31, 2014
Variable Interest Entity [Line Items] |  |  |
Carrying amounts of assets | $ 5,700 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets |  | $ 5,000 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
Carrying amounts of liabilities | 5,500 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities |  | 4,800 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities
Number of variable interest entities consolidated | 1 mbi_NumberOfVariableInterestEntitiesConsolidated | 0 mbi_NumberOfVariableInterestEntitiesConsolidated |
Net realized gains (losses) related to the initial consolidation | 0 us-gaap_VariableInterestEntityInitialConsolidationGainOrLoss | 0 us-gaap_VariableInterestEntityInitialConsolidationGainOrLoss |
Advisory Services [Member] |  |  |
Variable Interest Entity [Line Items] |  |  |
Carrying amounts of assets |  |  | 751 us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets / us-gaap_StatementBusinessSegmentsAxis = mbi_AdvisoryServicesMember
Carrying amounts of liabilities |  |  | 754 us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities / us-gaap_StatementBusinessSegmentsAxis = mbi_AdvisoryServicesMember
Number of variable interest entities consolidated |  |  | 3 mbi_NumberOfVariableInterestEntitiesConsolidated / us-gaap_StatementBusinessSegmentsAxis = mbi_AdvisoryServicesMember
Variable Interest Entity Primary Beneficiary [Member] | Collateral Loan Obligations [Member] |  |  |
Variable Interest Entity [Line Items] |  |  |
Investments carried at fair value |  |  | $ 4 us-gaap_TradingSecurities / invest_InvestmentAxis = us-gaap_CollateralizedLoanObligationsMember / us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis = us-gaap_VariableInterestEntityPrimaryBeneficiaryMember

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- Definition Represents the number of VIEs consolidated during the period. + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Amount of investments in debt and equity securities and other forms of securities that provide ownership interests classified as trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26626-111562 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Trading Securities -URI http://asc.fasb.org/extlink&oid=6526789 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.4) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_TradingSecurities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5710-111685 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition The carrying amount of the consolidated Variable Interest Entity's liabilities included in the reporting entity's statement of financial position. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5710-111685 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The gain (loss) recognized on initial consolidation of a variable interest entity (VIE) when the VIE is not a business (as defined). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=28200181&loc=d3e5710-111685 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |