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Related Party Transactions (Narrative) (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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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 | 5 mbi_NumberOfSuretyBonds |
Surety bonds maximum exposure | $ 340 mbi_OutstandingExposureOnSuretyBonds |
Number of loans to executive officers or directors | 0 mbi_LoansToExecutiveOfficersOrDirectors | 0 mbi_LoansToExecutiveOfficersOrDirectors

X
- Definition Number of loans made to executive officers or directors of teh Company. + References No definition available. + Details
Name: | mbi_LoansToExecutiveOfficersOrDirectors
Namespace Prefix: | mbi_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Number of surety bonds executed to guarantee the payment obligations of the members of the Municipal Bond Insurance Association. + References No definition available. + Details
Name: | mbi_NumberOfSuretyBonds
Namespace Prefix: | mbi_
Data Type: | xbrli:positiveIntegerItemType
Balance Type: | na
Period Type: | duration

X
- Definition Maximum amount the Company would be required to pay on the surety bonds. + References No definition available. + Details
Name: | mbi_OutstandingExposureOnSuretyBonds
Namespace Prefix: | mbi_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

X
- Definition Percentage of ownership of the Company defined as principal owners. + References No definition available. + Details
Name: | mbi_PercentageOfVotingInterestsOfCompany
Namespace Prefix: | mbi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

X
- Details
Name: | us-gaap_RelatedPartyTransactionsAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration