Financial Guaranty Contracts Accounted for as Credit Derivatives (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2011	Jun. 30, 2011	Dec. 31, 2010
Financial Guaranty Contracts Accounted for as Credit Derivatives
Estimated remaining weighted average life of credit derivatives (in years)	4.5Y		4.9Y
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	$ 90,704		$ 109,771
Pooled corporate obligations
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	31.30%		31.70%
Current Subordination (as a percent)	28.60%		29.30%
Net Par Outstanding	66,236		79,933
CLOs/CBOs
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	32.70%		32.20%
Current Subordination (as a percent)	32.10%		30.40%
Net Par Outstanding	36,983		45,953
Maximum average obligor size (as a percent)	1.00%
Maximum exposure of one industry (as a percent)	10.00%
Synthetic investment grade pooled corporate
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	19.80%		19.20%
Current Subordination (as a percent)	18.40%		17.60%
Net Par Outstanding	13,379		14,905
Synthetic high yield pooled corporate
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	35.60%		39.40%
Current Subordination (as a percent)	30.50%		34.60%
Net Par Outstanding	6,027		8,249
Trust Preferred Securities Collateralized Debt Obligations Member
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	46.60%		46.80%
Current Subordination (as a percent)	31.80%		32.00%
Net Par Outstanding	4,613		5,757
Amount of transactions insured converted from CDS form to financial guaranty form		837.8
Market value collateralized debt obligations of corporate obligations
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	32.10%		36.00%
Current Subordination (as a percent)	25.00%		42.90%
Net Par Outstanding	5,234		5,069
U.S. RMBS
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	24.00%		23.10%
Current Subordination (as a percent)	31.70%		32.40%
Net Par Outstanding	8,858		9,776
Option ARM and Alt-A first lien
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	19.60%		19.70%
Current Subordination (as a percent)	13.00%		17.00%
Net Par Outstanding	4,251		4,767
Subprime
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	30.10%		27.90%
Current Subordination (as a percent)	54.00%		50.40%
Net Par Outstanding	4,132		4,460
Prime first lien
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	10.90%		10.90%
Current Subordination (as a percent)	9.10%		10.30%
Net Par Outstanding	410		468
Closed end second lien and HELOCs
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	65		81
CMBS
Net Par Outstanding on Credit Derivatives
Original Subordination (as a percent)	33.90%		29.80%
Current Subordination (as a percent)	40.10%		31.30%
Net Par Outstanding	4,480		6,751
Other:
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	11,130		13,311
Pooled infrastructure transactions
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	3,400
Number of transactions	3
Remaining other CDS
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	$ 7,700