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v3.19.3.a.u2

Significant Accounting Policies (Narrative) (Detail) $ in Millions | 12 Months Ended
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Dec. 31, 2018 USD ($)
Reclassification Adjustment Increase Decrease In Insurance Loss Recoverable | $ 31

X
- Definition Represents Reclassification Adjustment Increase Decrease In Insurance Loss Recoverable. + References No definition available. + Details
Name: | mbi_ReclassificationAdjustmentIncreaseDecreaseInInsuranceLossRecoverable
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration