Loss And Loss Adjustment Expense Reserves (Summary Of Credit Impairments Related To Derivatives) (Narrative) (Details) (USD $)	12 Months Ended
	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%