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Investments (Significant Inputs Used In Measurement Of Credit Losses On Securities) (Detail) | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2011 | Dec. 31, 2010
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items] |  |  |  |  |  |
Expected size of losses, Weighted average | 87.67% | [1],[2] | 55.40% | [1],[2] | 52.20% | [1],[2]
Current subordination levels, Weighted average | 0.00% | [2],[3] | 1.53% | [2],[3] | 4.50% | [2],[3]
Prepayment speed (annual constant prepayment rate), Weighted average | 11.93% | [2],[4] | 12.67% | [2],[4] | 9.45% | [2],[4]
Minimum [Member] |  |  |  |  |  |
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items] |  |  |  |  |  |
Expected size of losses, Range | 12.13% | [1],[5] | 2.48% | [1],[5] | 0.21% | [1],[5]
Current subordination levels, Range | 0.00% | [3],[5] | 0.00% | [3],[5] | 0.00% | [3],[5]
Prepayment speed (annual constant prepayment rate), Range | 0.00% | [4],[5] | 0.00% | [4],[5] | 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] | 100.00% | [1],[5] | 100.00% | [1],[5]
Current subordination levels, Range | 0.00% | [3],[5] | 35.46% | [3],[5] | 42.16% | [3],[5]
Prepayment speed (annual constant prepayment rate), Range | 30.91% | [4],[5] | 100.00% | [4],[5] | 40.20% | [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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- 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
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- 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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Data Type: | num:percentItemType
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- 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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- 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 (annual constant prepayment rate) represents high and low points of lifetime vectors of constant prepayment rates, expressed as a percentage of total current outstanding 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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- Details
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