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Loss And Loss Adjustment Expense Reserves (Summary Of Credit Impairments Related To Derivatives) (Narrative) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2011 | Dec. 31, 2010
Loss And Loss Adjustment Expense Reserves [Line Items] |  |
Discount rate of credit impairments | 5.59% |
Additional estimated credit impairment on CMBS derivatives | $ 1,600,000,000 |
Aggregate ITD credit impairment on CMBS derivatives | 2,800,000,000 |
Number of structured CMBS commutations | 54 |
Gross exposure commuted on CMBS insured derivatives | 28,700,000,000 |
Life of loans, minimum | 5Y |
Life of loans, maximum | 6Y |
Maximum loans with debt service coverage ratio | $ 75,000,000 |
Debt service coverage ratio | 1.0 |
Percentage of financial statements received from properties held in pools |  | 82.00%
Commercial Mortgage Backed [Member] | Maximum [Member] |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |
Percentage of loss on liquidation | 2.00% |
Commercial Mortgage Backed [Member] | Minimum [Member] |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |
Percentage of loss on liquidation | 1.00% |

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