Loss And Loss Adjustment Expense Reserves (RMBS Recoveries) (Narrative) (Detail) (USD $)	12 Months Ended						12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Jan. 07, 2013 Bank Of America Settlement With Fannie Mae [Member]	Dec. 31, 2012 Second-Lien RMBS Reserves [Member]	Dec. 31, 2012 RMBS And HELOC And CES [Member]	Dec. 31, 2012 Second-Lien RMBS Ineligible Mortgage Loans [Member] Minimum [Member]	Dec. 31, 2012 Second-Lien RMBS Ineligible Mortgage Loans [Member] Maximum [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Total estimated recoveries from ineligible loans					$ 3,600,000,000
Insurance loss recoverable					2,500,000,000
Loan repurchase commitments					1,100,000,000
Federal statutory rate	35.00%	35.00%	35.00%
Estimated insurance recoveries, after income taxes	2,300,000,000	2,000,000,000
Percentage of recognized estimated recoveries in relationship to consolidated total shareholder's equity	73.00%	119.00%
Sum of probabilities assigned to all scenarios						100.00%
Current risk-free rate associated with underlying transaction, range, minimum						0.90%
Current risk-free rate associated with underlying transaction, range, maximum						1.70%
Remaining weighted average contract period (in years)	8 years 8 months 12 days	8 years 9 months 18 days					5 years 7 months 6 days	10 years 2 months 12 days
Contract claims on ineligible mortgage loans, incurred loss on transactions						5,100,000,000
Aggregate of estimated potential recoveries on put-back claims, gross of income taxes	3,600,000,000
Bank of America repurchase requests resolved			8,000,000,000	10,300,000,000
Assured Guaranty favorable ruling on breach of mortgage representations	$ 90,000,000