Loss And Loss Adjustment Expense Reserves (RMBS Recoveries) (Narrative) (Detail) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2012	Dec. 31, 2010
Loss And Loss Adjustment Expense Reserves [Line Items]
Federal statutory rate	35.00%	35.00%
Estimated insurance recoveries, after income taxes	$ 2,600,000,000	$ 2,300,000,000	$ 2,300,000,000
Percentage of recognized estimated recoveries in relationship to consolidated total shareholder's equity	78.00%	73.00%	73.00%
Remaining weighted average contract period (in years)	8 years 9 months 18 days		8 years 8 months 12 days
Aggregate of estimated potential recoveries on put-back claims, gross of income taxes	4,000,000,000
Bank of America repurchase requests resolved				8,000,000,000
Assured Guaranty favorable ruling on breach of mortgage representations	90,000,000
Second-Lien RMBS Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Total estimated recoveries from ineligible loans	4,000,000,000
Insurance loss recoverable	2,800,000,000
Loan repurchase commitments	1,200,000,000
RMBS And HELOC And CES [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Sum of probabilities assigned to all scenarios	100.00%
RMBS And HELOC And CES [Member] | Excluding Bank of America and Flagstar [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Current risk-free rate associated with underlying transaction, range, minimum	0.90%
Current risk-free rate associated with underlying transaction, range, maximum	2.00%
Contract claims on ineligible mortgage loans, inception to date incurred loss on transactions	$ 2,100,000,000
Second-Lien RMBS Ineligible Mortgage Loans [Member] | Minimum [Member] | Excluding Bank of America and Flagstar [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Remaining weighted average contract period (in years)	5 years 6 months
Second-Lien RMBS Ineligible Mortgage Loans [Member] | Maximum [Member] | Excluding Bank of America and Flagstar [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Remaining weighted average contract period (in years)	11 years 1 month 6 days