Financial Guaranty Contracts Accounted for as Credit Derivatives (Details) (USD $)	3 Months Ended		12 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Financial Guaranty Contracts Accounted for as Credit Derivatives
Estimated remaining weighted average life of credit derivatives (in years)	4 years		4 years 3 months 18 days
Net par of terminated CDS transactions	$ 200,000,000	$ 2,600,000,000
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	83,002,000,000		85,047,000,000
Pooled corporate obligations
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	32.00%		31.20%
Current Subordination (as a percent)	29.30%		28.90%
Net Par Outstanding	59,467,000,000		61,073,000,000
Collateralized loan obligation/collateral bond obligations
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	33.00%		32.60%
Current Subordination (as a percent)	32.20%		32.00%
Net Par Outstanding	34,272,000,000		34,567,000,000
Synthetic investment grade pooled corporate
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	20.80%		20.40%
Current Subordination (as a percent)	19.00%		18.70%
Net Par Outstanding	11,521,000,000		12,393,000,000
Synthetic high yield pooled corporate
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	35.70%		35.70%
Current Subordination (as a percent)	29.80%		30.30%
Net Par Outstanding	5,077,000,000		5,049,000,000
Trust preferred securities collateralized debt obligations
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	46.50%		46.60%
Current Subordination (as a percent)	31.60%		31.90%
Net Par Outstanding	4,475,000,000		4,518,000,000
Market value collateralized debt obligations of corporate obligations
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	34.90%		30.60%
Current Subordination (as a percent)	30.10%		28.90%
Net Par Outstanding	4,122,000,000		4,546,000,000
Maximum average obligor size (as a percent)	1.00%
Maximum exposure of one industry (as a percent)	10.00%
U.S. RMBS
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	23.50%		24.10%
Current Subordination (as a percent)	31.80%		32.20%
Net Par Outstanding	8,319,000,000		8,532,000,000
Option ARM and Alt-A first lien
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	19.20%		19.60%
Current Subordination (as a percent)	12.80%		13.60%
Net Par Outstanding	3,953,000,000		4,060,000,000
Subprime
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	29.40%		30.10%
Current Subordination (as a percent)	53.60%		53.90%
Net Par Outstanding	3,925,000,000		4,012,000,000
Prime first lien
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	10.90%		10.90%
Current Subordination (as a percent)	7.90%		8.40%
Net Par Outstanding	382,000,000		398,000,000
Closed end second lien and HELOCs
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	59,000,000		62,000,000
CMBS
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	33.50%		32.60%
Current Subordination (as a percent)	40.30%		38.90%
Net Par Outstanding	4,410,000,000		4,612,000,000
Other:
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	10,806,000,000		10,830,000,000
Pooled infrastructure transactions
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	3,200,000,000
Number of transactions	3
Remaining other CDS
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	$ 7,600,000,000