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Loss And Loss Adjustment Expense Reserves (Summary Of Credit Impairments Related To Derivatives) (Narrative) (Detail) (USD $)
12 Months Ended 36 Months Ended
Dec. 31, 2012
SecurityLoan
Dec. 31, 2012
SecurityLoan
Loss And Loss Adjustment Expense Reserves [Line Items]    
Discount rate of credit impairments 5.72%  
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 50,000,000  
Additional estimated credit impairments on CMBS derivatives 852,000,000  
Aggregate ITD credit impairment on CMBS derivatives   3,600,000,000
CMBS severities exceeded complete loss in certain instances 100.00%  
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%  
CRE CDO [Member]
   
Loss And Loss Adjustment Expense Reserves [Line Items]    
Losses and LAE $ 46,000,000