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Loss And Loss Adjustment Expense Reserves (Summary Of Credit Impairments Related To Derivatives) (Narrative) (Details) (USD $) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2012 loan | 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 | $ 295,000,000 |  |
Aggregate ITD credit impairment on CMBS derivatives | 3,100,000,000 |  |
Number of structured CMBS commutations | 65 |  |
Gross exposure commuted on CMBS insured derivatives | 32,900,000,000 |  |
Life of loans, minimum | 4Y |  |
Life of loans, maximum | 5Y |  |
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 |  | 31.00% | 53.00%
Proprietary performance data on CRE loans, amount | 80,000 |  |
Maximum Range [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Percentage of loss on liquidation | 2.00% |  |
Minimum Range [Member] |  |  |
Loss And Loss Adjustment Expense Reserves [Line Items] |  |  |
Percentage of loss on liquidation | 1.00% |  |

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