Loss And Loss Adjustment Expense Reserves (Summary Of Credit Impairments Related To Derivatives) (Narrative) (Detail) (USD $)	6 Months Ended
	Jun. 30, 2012 Security Loan
Loss And Loss Adjustment Expense Reserves [Line Items]
Discount rate of credit impairments	5.59%
Additional estimated credit impairment on CMBS derivatives	$ 454,000,000
Aggregate ITD credit impairment on CMBS derivatives	3,200,000,000
Number of structured CMBS commutations	65
Gross exposure commuted on CMBS insured derivatives	32,900,000,000
Maximum loans with debt service coverage ratio	$ 75,000,000
Debt service coverage ratio	1.0
Proprietary performance data on CRE loans, amount	80,000
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%