Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) (USD $)	3 Months Ended			3 Months Ended						3 Months Ended
	Mar. 31, 2013	Mar. 31, 2013 Variable Interest Entity [Member]	Dec. 31, 2012 Variable Interest Entity [Member]	Mar. 31, 2013 RMBS And HELOC And CES [Member]	Mar. 31, 2013 Financial Guarantees [Member]	Mar. 31, 2013 Financial Guarantees [Member] Variable Interest Entity [Member]	Mar. 31, 2013 Loss And LAE Reserves [Member]	Dec. 31, 2012 Loss And LAE Reserves [Member]	Mar. 31, 2013 Second-Lien RMBS Reserves [Member]	Mar. 31, 2013 ABS CDO Portfolio [Member]	Mar. 31, 2013 Classified [Member] RMBS And HELOC And CES [Member] Reviewed [Member]	Mar. 31, 2013 Classified [Member] Recoveries On Unpaid Losses [Member] RMBS And HELOC And CES [Member] Reviewed [Member]	Mar. 31, 2013 Insured Issues Designated As "Classified List" [Member] RMBS And HELOC And CES [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
ABS CDO exposure decrease since December 31,2007										89.00%
Aggregate ABS CDO Loss And LAE Reserves					$ 107,000,000	$ 234,000,000
Policy holder (Benefits) and Claims Incurred	(194,000,000)									(37,000,000)
Elimination of loss and LAE expense as result of consolidating VIE										2,000,000
Reduction of excess interest cash flows				94,000,000
Reserves for expected future claim payments							1,100,000,000	1,200,000,000
Expected recoveries of future payments							289,000,000	332,000,000
Paid losses and LAE, net of reinsurance and collections	102,000,000			84,000,000
Increase in insurance loss recoverable related to paid losses	229,000,000
Number of insured issues reviewed											30		35
Number of insured issues reviewed with recorded recoveries												27
Number of transactions received consideration and excluded from loan files reviewed with potential recoveries including one Alt-A	2
Weighted average risk-free rate used to discount claim liability	1.50%
Total estimated recoveries from ineligible loans									4,000,000,000
Insurance loss recoverable from ineligible loans									2,800,000,000
Loan repurchase commitments		$ 1,176,000,000	$ 1,086,000,000						$ 1,200,000,000