Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Details) (USD $)	12 Months Ended			12 Months Ended	48 Months Ended						12 Months Ended						12 Months Ended		12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Insured Issues Designated As Classified List [Member]	Dec. 31, 2011 ABS CDO Portfolio [Member]	Dec. 31, 2011 ABS CDO Portfolio [Member]	Dec. 31, 2011 Reviewed [Member] Classified [Member]	Dec. 31, 2011 Loss And LAE Reserves [Member]	Dec. 31, 2010 Loss And LAE Reserves [Member]	Dec. 31, 2011 Insurance Loss Recoverable [Member] Classified [Member]	Dec. 31, 2010 Insurance Loss Recoverable [Member] Classified [Member]	Dec. 31, 2011 Recoveries On Unpaid Losses [Member] Reviewed [Member] Classified [Member]	Dec. 31, 2011 Second-Lien RMBS [Member]	Dec. 31, 2010 Second-Lien RMBS [Member]	Dec. 31, 2011 Second-Lien RMBS [Member] Loan Repurchase Commitments [Member]	Dec. 31, 2011 Ineligible Mortgage Loans [Member]	Dec. 31, 2010 Ineligible Mortgage Loans [Member]	Dec. 31, 2011 RMBS And HELOC And CES [Member]	Dec. 31, 2011 Financial Guarantees [Member]	Dec. 31, 2011 Insured Credit Derivatives [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
ABS CDO exposure decrease since December 31, 2007					83.00%
Aggregate ABS CDO loss and LAE reserves																		$ 171,000,000
ABS CDO aggregate loss and LAE benefit																			551,000,000
Estimated insurance recoveries									3,032,000,000	2,531,000,000		3,100,000,000		835,000,000	3,100,000,000	2,500,000,000
Insurance loss recoverable on ineligible loans												2,000,000,000	1,700,000,000
Insurance loss recoverable									3,046,000,000	2,531,000,000		2,000,000,000
Estimated insured credit derivative impairments and LAE reserves, net of reinsurance and recoveries				444,000,000
Losses and loss adjustment				32,000,000
Elimination of loss and LAE expense as a result of consolidating VIEs				44,000,000
Change in recoveries of actual and expected payments related to ineligible mortgages																	448,000,000
Excess interest cash flows from securitizations																	68,000,000
Total paid losses, net of reinsurance and collections	729,000,000											507,000,000
Increase in insurance loss recoverable related to paid losses	507,000,000
Reserves for expected future claim payments							1,400,000,000	2,000,000,000
Expected recoveries of future payments							$ 562,000,000	$ 896,000,000
Number of insured issues reviewed			34			29
Number of insured issues reviewed with recorded recoveries											26
Weighted average risk-free rate used to discount claim liability	1.53%	2.73%