Loss And Loss Adjustment Expense Reserves (RMBS Recoveries) (Narrative) (Detail) (USD $)	3 Months Ended	6 Months Ended		12 Months Ended
	Jun. 30, 2012	Jun. 30, 2012	Jun. 30, 2011	Dec. 31, 2011	Jul. 31, 2012	Apr. 30, 2011	Dec. 31, 2010
Loss And Loss Adjustment Expense Reserves [Line Items]
Insurance loss recoverable	$ 3,212,000,000	$ 3,212,000,000		$ 3,046,000,000
Federal statutory rate	35.00%	35.00%	35.00%	35.00%
Estimated insurance recoveries, after income taxes	2,100,000,000	2,100,000,000		2,000,000,000
Percentage of recognized estimated recoveries in relationship to consolidated total shareholder's equity	83.00%	83.00%		119.00%
Aggregate of estimated potential recoveries, gross of income taxes	3,200,000,000	3,200,000,000
Settlement received for assured guaranty put-back claims						1,100,000,000
Settlement received by assured guaranty from bank of america for put back claims					375,000,000
Bank of America repurchase requests resolved							8,000,000,000
Second-Lien RMBS Reserves [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Total estimated recoveries from ineligible loans	3,200,000,000	3,200,000,000
Insurance loss recoverable	2,200,000,000	2,200,000,000
Loan repurchase commitments	1,000,000,000	1,000,000,000
First And Second Lien RMBS [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Breach rate of ineligible mortgage loans reviewed	80.00%	80.00%
RMBS And HELOC And CES [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Sum of probabilities assigned to all scenarios	100.00%	100.00%
Current risk-free rate associated with underlying transaction, range, minimum	0.70%	0.70%
Current risk-free rate associated with underlying transaction, range, maximum	1.60%	1.60%
Contract claims on ineligible mortgage loans, incurred loss on transactions	$ 4,900,000,000	$ 4,900,000,000