Loss And Loss Adjustment Expense Reserves (RMBS Recoveries) (Narrative) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Loss And Loss Adjustment Expense Reserves [Line Items]
Federal statutory rate	35.00%	35.00%	35.00%
Estimated insurance recoveries, after income taxes	$ 2,000,000,000	$ 1,600,000,000
Percentage of recognized estimated recoveries in relationship to consolidated total shareholder's equity	119.00%	58.00%
Percentage of ineligible mortgage loan recoveries of statutory capital	89.00%	59.00%
Contract claims on ineligible mortgages	4,600,000,000
Aggregate of estimated potential recoveries	3,100,000,000
Settlement received by assured guaranty from Bank Of America for put-back claims	1,100,000,000
Bank of America repurchase requests resolved		8,000,000,000
Second-Lien RMBS [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Estimated insurance recoveries	3,100,000,000
Insurance loss recoverable	2,000,000,000
Loan repurchase commitments	1,100,000,000
Estimated insurance recoveries, after income taxes	$ 2,000,000,000	$ 1,600,000,000
First And Second Lien RMBS [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Percentage of ineligible mortgage loans	80.00%
RMBS [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Sum of probabilities assigned to all scenarios	100.00%
RMBS And HELOC And CES [Member]
Loss And Loss Adjustment Expense Reserves [Line Items]
Current risk-free rate associated with underlying transaction, range, minimum	0.92%
Current risk-free rate associated with underlying transaction, range, maximum	1.92%