Loss And Loss Adjustment Expense Reserves (Loss And LAE Activity) (Narrative) (Detail) (USD $)	6 Months Ended		6 Months Ended	54 Months Ended		6 Months Ended			6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Jun. 30, 2012 ABS CDO Portfolio [Member]	Jun. 30, 2012 ABS CDO Portfolio [Member]	Jun. 30, 2012 Insured Issues Designated As "Classified List" [Member]	Jun. 30, 2012 RMBS And HELOC And CES [Member]	Jun. 30, 2012 RMBS And HELOC And CES [Member] Classified [Member] Reviewed [Member]	Jun. 30, 2012 RMBS And HELOC And CES [Member] Insured Issues Designated As "Classified List" [Member]	Jun. 30, 2012 Second-Lien RMBS Reserves [Member]	Jun. 30, 2012 Ineligible Mortgage Loans [Member]	Jun. 30, 2012 Financial Guarantee [Member]	Jun. 30, 2012 Financial Guarantee [Member] Variable Interest Entity [Member]	Jun. 30, 2012 Loss And LAE Reserves [Member]	Dec. 31, 2011 Loss And LAE Reserves [Member]	Jun. 30, 2012 Recoveries On Unpaid Losses [Member] RMBS And HELOC And CES [Member] Classified [Member] Reviewed [Member]	Jun. 30, 2012 Insurance Loss Recoverable [Member]	Jun. 30, 2012 Insurance Loss Recoverable [Member] Classified [Member]	Dec. 31, 2011 Insurance Loss Recoverable [Member] Classified [Member]	Jun. 30, 2012 Loan Repurchase Commitments [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											$ 167,000,000	$ 244,000,000
Policy holder Benefits And Claims Incurred Net	159,000,000		2,000,000						67,000,000
Elimination Of Loss And LAE Expense As Result Of Consolidating VIE			17,000,000
Excess interest cash flows from securitizations						43,000,000
Total paid losses, net of reinsurance and collections	325,000,000								252,000,000
Increase in insurance loss recoverable related to paid losses	165,000,000
Reserves for expected future claim payments													1,300,000,000	1,400,000,000
Expected recoveries of future payments	508,000,000	562,000,000											508,000,000	562,000,000
Number of insured issues reviewed					34		29	34
Number of insured issues reviewed with recorded recoveries							29								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	1.30%
Total estimated recoveries from ineligible loans									$ 3,200,000,000	$ 3,200,000,000						$ 2,200,000,000	$ 3,185,000,000	$ 3,032,000,000	$ 1,000,000,000