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Loss And Loss Adjustment Expense Reserves (Summary Of Credit Impairments Related To Derivatives) (Narrative) (Details) (USD $)	6 Months Ended
	Jun. 30, 2011
Discount rate of credit impairments	5.93%
Additional estimated credit impairment on derivatives	$ 363,000,000
Aggregate credit impairment on derivatives	1,500,000,000
Number of structured derivatives	22
Exposed gross insured derivatives	10,300,000,000
Life of loans, minimum	6Y
Life of loans, maximum	7Y
Maximum loans with debt service coverage ratio	75,000,000
Debt service coverage ratio	1.0
Percentage of loans greater than ninety days	75.00%
Percentage of loans liquidated	65.00%
Percentage of loans modified	35.00%
Weighted loss estimate	$ 1,500,000,000
Percentage of properties held in pools	68.00%
Maximum Range [Member]
Percentage of loss on liquidation	2.00%
Minimum Range [Member]
Percentage of loss on liquidation	1.00%

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