Financial Guaranty Contracts Accounted for as Credit Derivatives (Details) (USD $)	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2011 Pooled corporate obligations	Dec. 31, 2010 Pooled corporate obligations	Dec. 31, 2011 Collateralized loan obligation/collateral bond obligations	Dec. 31, 2010 Collateralized loan obligation/collateral bond obligations	Dec. 31, 2011 Synthetic investment grade pooled corporate	Dec. 31, 2010 Synthetic investment grade pooled corporate	Dec. 31, 2011 Synthetic high yield pooled corporate	Dec. 31, 2010 Synthetic high yield pooled corporate	Dec. 31, 2011 Trust preferred securities collateralized debt obligations	Jun. 30, 2011 Trust preferred securities collateralized debt obligations	Dec. 31, 2010 Trust preferred securities collateralized debt obligations	Dec. 31, 2011 Market value collateralized debt obligations of corporate obligations	Dec. 31, 2010 Market value collateralized debt obligations of corporate obligations	Dec. 31, 2011 U.S. RMBS	Dec. 31, 2010 U.S. RMBS	Dec. 31, 2011 Option ARM and Alt-A first lien	Dec. 31, 2010 Option ARM and Alt-A first lien	Dec. 31, 2011 Subprime	Dec. 31, 2010 Subprime	Dec. 31, 2011 Prime first lien	Dec. 31, 2010 Prime first lien	Dec. 31, 2011 Closed end second lien and HELOCs	Dec. 31, 2010 Closed end second lien and HELOCs	Dec. 31, 2011 CMBS	Dec. 31, 2010 CMBS	Dec. 31, 2011 Other:	Dec. 31, 2010 Other:	Dec. 31, 2011 Pooled infrastructure transactions Transaction	Dec. 31, 2011 Remaining other CDS
Financial Guaranty Contracts Accounted for as Credit Derivatives
Estimated remaining weighted average life of credit derivatives (in years)	P4.3Y	P4.9Y
Net par of terminated CDS transactions	$ 11,500,000,000
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)			31.20%	31.70%	32.60%	32.20%	20.40%	19.20%	35.70%	39.40%	46.60%		46.80%	30.60%	36.00%	24.10%	23.10%	19.60%	19.70%	30.10%	27.90%	10.90%	10.90%			32.60%	29.80%
Current Subordination (as a percent)			28.90%	29.30%	32.00%	30.40%	18.70%	17.60%	30.30%	34.60%	31.90%		32.00%	28.90%	42.90%	32.20%	32.40%	13.60%	17.00%	53.90%	50.40%	8.40%	10.30%			38.90%	31.30%
Net Par Outstanding	85,047,000,000	109,771,000,000	61,073,000,000	79,933,000,000	34,567,000,000	45,953,000,000	12,393,000,000	14,905,000,000	5,049,000,000	8,249,000,000	4,518,000,000		5,757,000,000	4,546,000,000	5,069,000,000	8,532,000,000	9,776,000,000	4,060,000,000	4,767,000,000	4,012,000,000	4,460,000,000	398,000,000	468,000,000	62,000,000	81,000,000	4,612,000,000	6,751,000,000	10,830,000,000	13,311,000,000	3,100,000,000	7,700,000,000
Number of transactions																														3
Maximum average obligor size (as a percent)					1.00%
Maximum exposure of one industry (as a percent)					10.00%
Amount of transactions insured converted from CDS form to financial guaranty form												$ 837,800,000