Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) (USD $)	12 Months Ended				12 Months Ended							12 Months Ended	60 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Variable Interest Entity [Member]	Dec. 31, 2011 Variable Interest Entity [Member]	Dec. 31, 2012 RMBS And HELOC And CES [Member]	Dec. 31, 2012 Financial Guarantees [Member]	Dec. 31, 2012 Financial Guarantees [Member] Variable Interest Entity [Member]	Dec. 31, 2012 Loss And LAE Reserves [Member]	Dec. 31, 2011 Loss And LAE Reserves [Member]	Dec. 31, 2012 Second-Lien RMBS Reserves [Member]	Dec. 31, 2011 Second-Lien RMBS Reserves [Member]	Dec. 31, 2012 ABS CDO Portfolio [Member]	Dec. 31, 2012 ABS CDO Portfolio [Member]	Dec. 31, 2012 Classified [Member] RMBS And HELOC And CES [Member] Reviewed [Member]	Dec. 31, 2012 Classified [Member] Recoveries On Unpaid Losses [Member] RMBS And HELOC And CES [Member] Reviewed [Member]	Dec. 31, 2012 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													88.00%
Aggregate ABS CDO Loss And LAE Reserves						$ 143,000,000	$ 236,000,000
Policy holder (Benefits) and Claims Incurred	50,000,000											(21,000,000)
Elimination of loss and LAE expense as result of consolidating VIE												24,000,000
Reduction of excess interest cash flows					83,000,000
Reserves for expected future claim payments								1,200,000,000	1,400,000,000
Expected recoveries of future payments								332,000,000	562,000,000
Paid losses and LAE, net of reinsurance and collections	631,000,000				421,000,000
Increase in insurance loss recoverable related to paid losses	602,000,000
Number of insured issues reviewed														29		34
Number of insured issues reviewed with recorded recoveries															26
Number of transactions received consideration and excluded from loan files reviewed with potential recoveries	2
Weighted average risk-free rate used to discount claim liability	1.38%	1.53%
Total estimated recoveries from ineligible loans										3,600,000,000	3,100,000,000
Insurance Loss Recoverable from ineligible loans										2,500,000,000	2,000,000,000
Loan repurchase commitments			$ 1,086,000,000	$ 1,077,000,000						$ 1,100,000,000	$ 1,100,000,000