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Variable Interest Entities (Narrative) (Detail) $ in Billions | 3 Months Ended |
--- | --- | ---
Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($)
Variable Interest Entity [Line Items] |  |
Carrying amounts of assets | $ 3.2 | $ 3.2
Carrying amounts of liabilities | $ 2.3 | $ 2.3
Number of variable interest entities consolidated | 0 |
Variable Interest Entity Primary Beneficiary [Member] | Zohar Funds Bankruptcy Cases Debtors [Member] |  |
Variable Interest Entity [Line Items] |  |
Number of VIEs | 2 |

X
- Definition Number of consolidated VIEs as of the balance sheet date. + References No definition available. + Details
Name: | mbi_NumberOfConsolidatedVariableInterestEntities
Namespace Prefix: | mbi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

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- Definition Represents the number of VIEs consolidated during the period. + References No definition available. + Details
Name: | mbi_NumberOfVariableInterestEntitiesConsolidated
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://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=77990344&loc=d3e5710-111685 + Details
Name: | us-gaap_VariableInterestEntityConsolidatedCarryingAmountAssets
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://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=77990344&loc=d3e5710-111685 + Details
Name: | us-gaap_VariableInterestEntityConsolidatedCarryingAmountLiabilities
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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
Name: | us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_VariableInterestEntityPrimaryBeneficiaryMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MajorCustomersAxis=mbi_ZoharFundsBankruptcyCasesDebtorsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |