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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%