Loss And Loss Adjustment Expense Reserves (Summary Of Credit Impairments Related To Derivatives) (Narrative) (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013 securityloan	Jun. 30, 2012	Jun. 30, 2013 securityloan	Jun. 30, 2012
Loss And Loss Adjustment Expense Reserves [Line Items]
Discount rate of credit impairments			5.72%
Number of structured Cmbs commutations since 2010			84
Gross exposure commuted on Cmbs insured derivatives since 2010			$ 41,600,000,000
Loan balance greater than with debt service coverage ratio	75,000,000		75,000,000
Debt service coverage ratio			1.0
Proprietary performance data on CRE loans, amount from 1992 to 2011	80,000		80,000
Number of representative cohorts in commercial real estate proprietary model based on debt service coverage ratio ("DSCR") and property type.			168
Policyholder (Benefits) And Claims Incurred Net	188,000,000	62,000,000	(6,000,000)	159,000,000
Additional estimated credit impairment on Cmbs derivatives			399,000,000
Aggregate ITD credit impairment on Cmbs derivatives since 2010			4,000,000,000
Cmbs severities exceeded complete loss in certain instances			100.00%
Commercial Real Estate Collateralized Debt Obligations [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Policyholder (Benefits) And Claims Incurred Net			$ (38,000,000)
Minimum [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Percentage of loss on liquidation			1.00%
Maximum [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Percentage of loss on liquidation			2.00%