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Investments (Credit Losses Recognized In Earnings Related To OTTI Losses Recognized In Accumulated Other Comprehensive Income (Loss)) (Detail) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Other than temporary impairment credit lossesfor available for sale securities rollforward [Abstract] |  |  |  |  |  |
Beginning Balance | $ 175 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld |  | $ 197 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld |  | $ 341 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld |
Additions For Credit Loss Impairments Recognized In The Current Period On Securities Not Previous Impaired | 15 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment |  | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment |  | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsNoPreviousImpairment |
Additions For Credit Loss Impairments Recognized In The Current Period On Securities Previously Impaired | 1 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses |  | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses |  | 8 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsAdditionsAdditionalCreditLosses |
Additions For Credit Loss Impairments Recognized in Prior Periods For Securities That Were Reimpaired With Non Credit Component In Current Period | 0 mbi_AdditionsForCreditLossImpairmentsRecognizedInPriorPeriodsForSecuritiesThatWereReimpairedWithNonCreditComponentInCurrentPeriod |  | 0 mbi_AdditionsForCreditLossImpairmentsRecognizedInPriorPeriodsForSecuritiesThatWereReimpairedWithNonCreditComponentInCurrentPeriod |  | 0 mbi_AdditionsForCreditLossImpairmentsRecognizedInPriorPeriodsForSecuritiesThatWereReimpairedWithNonCreditComponentInCurrentPeriod |
Reductions For Credit Loss Impairments Previously Recognized On Securities Sold During The Period | (174)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold |  | (16)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold |  | (41)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsSecuritiesSold |
Reductions For Credit Loss Impairments Previously Recognized On Securities Impaired To Fair Value During The Period | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsChangeInStatus | [1] | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsChangeInStatus | [1] | (111)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsChangeInStatus | [1]
Reductions For Increases In Cash Flows Expected To Be Collected Over The Remaining Life Of The Security | (1)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows |  | (6)us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows |  | 0 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsReductionsCashFlows |
Ending Balance | $ 16 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld |  | $ 175 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld |  | $ 197 us-gaap_OtherThanTemporaryImpairmentCreditLossesRecognizedInEarningsCreditLossesOnDebtSecuritiesHeld |
[1] | Represents circumstances where the Company determined in the current period that it intends to sell the security or it is more likely than not that it will be required to sell the security before recovery of the security's amortized cost.

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- Definition Additions For Credit Loss Impairments Recognized in Prior Periods For Securities That Were Reimpaired With Non Credit Component In Current Period + References No definition available. + Details
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- Definition This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which a previous OTTI was recognized and the investor does not intend to sell the debt security and it is not more likely than not that the investor will be required to sell the debt security before recovery of its amortized cost basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (e) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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- Definition This item represents an increase to the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held for which no such other than temporary impairment (OTTI) was previously recognized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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- Definition The cumulative amount of credit losses recognized in earnings related to debt securities held for which a portion of an other than temporary impairment (OTTI) was recognized in other comprehensive income (a component of shareholders' equity). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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- Definition This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security held due to increases in cash flows expected to be collected and recognized over the remaining life of the security. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (f) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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- Definition This item represents reductions in the amount of cumulative losses recognized in earnings for other than temporary impairments (OTTI) for which a portion of the impairment had been recognized in other comprehensive income (a component of shareholders' equity) due to a change in status of the impaired security because of a change in the entity's intent to either sell the impaired security or it has become more likely than not that the entity will be required to sell the security before recovery of its cost basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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- Definition This item represents reductions in the cumulative amount of credit losses recognized in earnings for an other than temporary impairment (OTTI) of a debt security because of the sale of impaired securities during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 8 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27340-111563 + Details
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