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Recent Accounting Pronouncements (Narrative) (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2017 USD ($)
Accounting Policies [Abstract] |
Estimated reclassification of cumulative effect accumulated other comprehensive income to retained earnings for equity investments. | $ 2.0
Estimated reclassification of cumulative effect retained earnings to accumulated other comprehensive income for financial liabilities | $ 162.0

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- Definition Cumulative equity adjustment for the adoption of ASU 2016-01. + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
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Period Type: | duration

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

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