Recent Accounting Pronouncements (Narrative) (Detail) $ in Millions	9 Months Ended
Sep. 30, 2017 USD ($)
Accounting Policies [Abstract]
Estimated reclassification of cumulative effect accumulated other comprehensive income to retained earnings for equity investments.	$ 1.0

Cumulative equity adjustment for the adoption of ASU 2016-01.

No definition available.

Name:	mbi_EstimatedReclassificationOfCumulativeEffectAccumulatedOtherComprehensiveIncomeToRetainedEarningsForEquityInvestments
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration