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Investments (Summary Of Significant Inputs Considered In Determining The Measurement Of Credit Losses On Securities) (Detail) | 9 Months Ended
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Sep. 30, 2012
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items] |  |
Expected size of losses, Weighted average | 88.48% | [1],[2]
Current subordination levels, Weighted average | 0.00% | [2],[3]
Prepayment speed (annual constant prepayment rate), Weighted average | 12.16% | [2],[4]
Minimum [Member] |  |
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items] |  |
Expected size of losses, Range | 12.13% | [1],[5]
Current subordination levels, Range | 0.00% | [3],[5]
Prepayment speed (annual constant prepayment rate), Range | 0.00% | [4],[5]
Maximum [Member] |  |
Other Than Temporary Impairment Credit Losses Recognized In Earnings [Line Items] |  |
Expected size of losses, Range | 97.70% | [1],[5]
Current subordination levels, Range | 0.00% | [3],[5]
Prepayment speed (annual constant prepayment rate), Range | 30.91% | [4],[5]
[1] | Represents future expected credit losses on impaired assets expressed as a percentage of total outstanding balance.
[2] | Calculated by weighting the relevant input/assumption for each individual security by the outstanding notional of the security.
[3] | Represents current level of credit protection (subordination) for the securities, expressed as a percentage of the balance of the collateral group backing the bond.
[4] | Values represent high and low points of lifetime vectors of constant prepayment rates.
[5] | Represents the range of inputs/assumptions based upon the individual securities within each category.

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- Definition Represents the range of inputs/assumptions used to calculate current level of credit protection (subordination) for the securities, expressed as a percentage of total current underlying loan balance + References No definition available. + Details
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X
- Definition Represents the range of inputs/assumptions used to calculate future expected credit losses on impaired assets expressed as a percentage of total current outstanding balance + References No definition available. + Details
Name: | mbi_AssumptionForMeasurementOfExpectedSizeOfLossesRange
Namespace Prefix: | mbi_
Data Type: | num:percentItemType
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X
- Definition Represents the range of inputs/assumptions used to calculate the high and low points of lifetime vectors of constant prepayment rates + References No definition available. + Details
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Namespace Prefix: | mbi_
Data Type: | num:percentItemType
Balance Type: | na
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X
- Definition Weighted average current subordination levels represents current level of credit protection (subordination) for the securities, expressed as a percentage of total current underlying loan balance calculated by weighting the relevant assumption for each individual security by current outstanding notional of the security. + References No definition available. + Details
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Namespace Prefix: | mbi_
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- Definition Weighted average expected size of losses represents future expected credit losses on impaired assets expressed as a percentage of total current outstanding balance calculated by weighting the relevant assumption for each individual security by current outstanding notional of the security. + References No definition available. + Details
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- Definition Weighted average prepayment speed represents high and low points of lifetime vectors of constant prepayment rates calculated by weighting the relevant assumption for each individual security by current outstanding notional of the security. + References No definition available. + Details
Name: | mbi_WeightedAverageAssumptionForMeasurementOfPrepaymentSpeed
Namespace Prefix: | mbi_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Details
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