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Loss And Loss Adjustment Expense Reserves (Summary Of Credit Impairments Related To Derivatives) (Narrative) (Detail) (USD $) | 9 Months Ended | 33 Months Ended
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Sep. 30, 2012 SecurityLoan | Sep. 30, 2012 SecurityLoan
Loss And Loss Adjustment Expense Reserves [Line Items] |  |
Discount rate of credit impairments | 5.59% |
Number of structured CMBS commutations |  | 66
Gross exposure commuted on CMBS insured derivatives |  | $ 33,100,000,000
Loan balance greater than with debt service coverage ratio | 75,000,000 |
Debt service coverage ratio | 1.0 |
Proprietary performance data on CRE loans, amount | 80,000 | 80,000
Representative Cohorts based on DSCR and property type | 168 |
Losses and LAE | 330,000,000 |
Additional estimated credit impairments on CMBS derivatives | 502,000,000 |
Aggregate ITD credit impairment on CMBS derivatives |  | 3,300,000,000
CMBS severities exceeded complete loss in certain instances | 100.00% |
CRE CDO [Member] |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |
Losses and LAE | $ 87,000,000 |
Maximum [Member] |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |
Percentage of loss on liquidation | 2.00% |
Minimum [Member] |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |
Percentage of loss on liquidation | 1.00% |

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