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Long-Term Incentive Plans (Outstanding And Exercisable Stock Options) (Detail) (USD $) In Millions, except Share data, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Number of Options, Outstanding	2,548,000.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	3,715,406.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	5,963,268.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber	6,425,073.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber
Number of Options, Exercisable	1,798,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	1,752,311us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber	1,751,173us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber
Weighted Average Remaining Contractual Life in Years, Stock Options Outstanding	1 year 6 months 29 days
Weighted Average Exercise Price, Stock Options Outstanding	$ 10.9888us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	$ 20.6883us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	$ 19.0485us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice	$ 21.333us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice
Weighted Average Exercise Price, Stock Options Exercisable	$ 13.466us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice	$ 38.512us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice	$ 45.0174us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice
Aggregate Intrinsic Value, Stock Options Outstanding	$ 11us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue
Aggregate Intrinsic Value, Stock Options Exercisable	8us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1
$4.02-$57.71 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Average Exercise Price, Minimum	$ 4.02us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FourPointZeroTwoToFiftySevenPointSevenOneMember
Range of Average Exercise Price, Maximum	$ 57.71us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FourPointZeroTwoToFiftySevenPointSevenOneMember
Number of Options, Outstanding	2,287,500.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FourPointZeroTwoToFiftySevenPointSevenOneMember
Number of Options, Exercisable	1,537,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FourPointZeroTwoToFiftySevenPointSevenOneMember
Weighted Average Remaining Contractual Life in Years, Stock Options Outstanding	1 year 8 months 12 days
Weighted Average Exercise Price, Stock Options Outstanding	$ 5.3922us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FourPointZeroTwoToFiftySevenPointSevenOneMember
Weighted Average Exercise Price, Stock Options Exercisable	$ 5.5591us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FourPointZeroTwoToFiftySevenPointSevenOneMember
Aggregate Intrinsic Value, Stock Options Outstanding	11us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FourPointZeroTwoToFiftySevenPointSevenOneMember
Aggregate Intrinsic Value, Stock Options Exercisable	$ 8us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1 / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FourPointZeroTwoToFiftySevenPointSevenOneMember
$58.84-$70.86 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Range of Average Exercise Price, Minimum	$ 58.84us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FiftyEightPointEightFourToSeventyPointEightSixMember
Range of Average Exercise Price, Maximum	$ 70.86us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FiftyEightPointEightFourToSeventyPointEightSixMember
Number of Options, Outstanding	260,500.00us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FiftyEightPointEightFourToSeventyPointEightSixMember
Number of Options, Exercisable	260,500us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FiftyEightPointEightFourToSeventyPointEightSixMember
Weighted Average Remaining Contractual Life in Years, Stock Options Outstanding	0 years 6 months 18 days
Weighted Average Exercise Price, Stock Options Outstanding	$ 60.1333us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FiftyEightPointEightFourToSeventyPointEightSixMember
Weighted Average Exercise Price, Stock Options Exercisable	$ 60.1333us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice / us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis = mbi_FiftyEightPointEightFourToSeventyPointEightSixMember
Options Exercisable [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Weighted Average Remaining Contractual Life in Years, Stock Options Outstanding	1 year 4 months 2 days
Options Exercisable [Member] | $4.02-$57.71 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Weighted Average Remaining Contractual Life in Years, Stock Options Outstanding	1 year 5 months 19 days
Options Exercisable [Member] | $58.84-$70.86 [Member]
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Weighted Average Remaining Contractual Life in Years, Stock Options Outstanding	0 years 6 months 18 days

X

- Definition

Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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Number of options outstanding, including both vested and non-vested options.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

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The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

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