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Related Party Transactions (Narrative) (Detail) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
Dec. 31, 2014
securityloan
policy
Dec. 31, 2013
securityloan
Related Party Transactions [Abstract]    
Principal ownership percentage 10.00%mbi_PercentageOfVotingInterestsOfCompany  
Number of executed surety bonds 5mbi_NumberOfSuretyBonds  
Surety bonds maximum exposure $ 340mbi_OutstandingExposureOnSuretyBonds  
Number of loans to executive officers or directors 0mbi_LoansToExecutiveOfficersOrDirectors 0mbi_LoansToExecutiveOfficersOrDirectors