Investments (Average Credit Ratings And Fair Value and Percentage Of Guaranteed ABS MBS And Corporate Obligations Below Investment Grade) (Detail) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Asset-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 101
Percentage	65.00%
Mortgage-Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fair Value	$ 113
Percentage	96.00%

Fair Value of Securities rated below investment grade without giving effect to the benefit of guarantees provided by financial guarantors.

No definition available.

Name:	mbi_FairValueOfSecuritiesRatedBelowInvestmentGradeWithoutGivingEffectToBenefitOfGuaranteesProvidedByFinancialGuarantors
Namespace Prefix:	mbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant