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Investment Income And Gains And Losses (Significant Inputs Used In Schedule Of Measurement Of Credit Loss Component Recognized In Earnings For Each Significant Class Of ABS) (Detail)
 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]    
Expected size of losses, Weighted average 88.48% [1],[2] 64.58% [1],[2]
Current subordination levels, Weighted average 0.00% [2],[3] 0.00% [2],[3]
Prepayment speed (annual constant prepayment rate), Weighted average 12.16% [2],[4] 15.62% [2],[4]
Minimum [Member]
    
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]    
Expected size of losses, Range 12.13% [1],[5] 1.59% [1],[5]
Current subordination levels, Range 0.00% [3],[5] 0.00% [3],[5]
Prepayment speed (annual constant prepayment rate), Range 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]
Current subordination levels, Range 0.00% [3],[5] 29.46% [3],[5]
Prepayment speed (annual constant prepayment rate), Range 30.91% [4],[5] 100.00% [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.