Loss And Loss Adjustment Expense Reserves (Summary Of Credit Impairments Related To Derivatives) (Narrative) (Detail) (USD $)	3 Months Ended	39 Months Ended
	Mar. 31, 2013 SecurityLoan	Mar. 31, 2013 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
Policy holder (Benefits) and Claims Incurred	(194,000,000)
Additional estimated credit impairments on CMBS derivatives	290,000,000
Aggregate ITD credit impairment on CMBS derivatives		3,900,000,000
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]
Policy holder (Benefits) and Claims Incurred	$ (8,000,000)