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Variable Interest Entities (Narrative) (Detail) $ in Millions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2019 USD ($) | Dec. 31, 2018 USD ($)
Variable Interest Entity [Line Items] |  |
Carrying amounts of assets | $ 2,400 | $ 1,700
Carrying amounts of liabilities | $ 2,600 | $ 1,700
Variable Interest Entity Primary Beneficiary [Member] |  |
Variable Interest Entity [Line Items] |  |
Number of variable interest entities deconsolidated | 0 |
Variable Interest Entity Primary Beneficiary [Member] | U S Public Finance Insurance [Member] |  |
Variable Interest Entity [Line Items] |  |
Number of variable interest entities consolidated | 7 |
Net realized gains (losses) related to the initial consolidation | $ (42) |

X
- 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: | duration

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

X
- 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5710-111685 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (bb) -URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5710-111685 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- 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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 810 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=108788376&loc=d3e5710-111685 + Details
Name: | us-gaap_VariableInterestEntityInitialConsolidationGainOrLoss
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_VariableInterestEntityLineItems
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: |

X
- Details
Name: | us-gaap_StatementBusinessSegmentsAxis=mbi_USPublicFinanceInsuranceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |