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Recent Accounting Pronouncements (Narrative) (Detail) $ in Millions | 3 Months Ended
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Mar. 31, 2017 USD ($)
Accounting Policies [Abstract] |
Estimated reclassification of the cumulative effect from accumulated other comprehensive income to retained earnings for equity investments. | $ 0.3
Estimated reclassification of the cumulative effect from retained earning to accumulated other comprehensive income for financial liabilities | $ 100.0

X
- Definition Estimated cumulative equity adjustment for the adoption of ASU 2016-01. + References No definition available. + Details
Name: | mbi_EstimatedReclassificationOfCumulativeEffectAccumulatedOtherComprehensiveIncomeToRetainedEarningsForEquityInvetments
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition Estimated cumulative equity adjustment for the adoption of ASU 2016-01. + References No definition available. + Details
Name: | mbi_EstimatedReclassificationOfCumulativeEffectRetainedEarningsToAccumulatedOtherComprehensiveIncomeForFinancialLiabilities
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- References No definition available. + Details
Name: | us-gaap_AccountingPoliciesAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration