Financial Guaranty Contracts Accounted for as Credit Derivatives (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Financial Guaranty Contracts Accounted for as Credit Derivatives
Estimated remaining weighted average life of credit derivatives (in years)	4 years 1 month 6 days		3 years 8 months 12 days
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	$ 59,109		$ 70,781
Original Subordination (as a percent)	0.00%	[1]
Current Subordination (as a percent)	0.00%	[1]	0.00%	[1]
Pooled corporate
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	40,050		50,120
Original Subordination (as a percent)	31.90%	[1]	31.70%	[1]
Current Subordination (as a percent)	30.90%	[1]	30.40%	[1]
Collateralized loan obligation/collateral bond obligations
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	20,858		29,142
Original Subordination (as a percent)	32.40%	[1]	32.80%	[1]
Current Subordination (as a percent)	34.20%	[1]	33.30%	[1]
Synthetic investment grade pooled corporate
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	9,716		9,658
Original Subordination (as a percent)	21.60%	[1]	21.60%	[1]
Current Subordination (as a percent)	19.70%	[1]	19.70%	[1]
Synthetic high yield pooled corporate
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	2,690		3,626
Original Subordination (as a percent)	47.20%	[1]	35.00%	[1]
Current Subordination (as a percent)	41.10%	[1]	30.30%	[1]
Trust Preferred Securities Collateralized Debt Obligations
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	3,673		4,099
Original Subordination (as a percent)	45.90%	[1]	46.50%	[1]
Current Subordination (as a percent)	33.60%	[1]	32.70%	[1]
Market value collateralized debt obligations of corporate obligations
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	3,113		3,595
Original Subordination (as a percent)	31.20%	[1]	30.10%	[1]
Current Subordination (as a percent)	31.90%	[1]	32.00%	[1]
Maximum average obligor size (as a percent)	1.00%
Maximum exposure of one industry (as a percent)	10.00%
Other Pooled Infrastructure Transactions [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	3,200
U.S. RMBS
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	6,365		7,257
Original Subordination (as a percent)	24.30%	[1]	24.20%	[1]
Current Subordination (as a percent)	28.30%	[1]	30.40%	[1]
Option ARM and Alt-A first lien
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	2,995		3,381
Original Subordination (as a percent)	19.80%	[1]	20.20%	[1]
Current Subordination (as a percent)	8.00%	[1]	10.40%	[1]
First mortgage, subprime
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	3,067		3,494
Original Subordination (as a percent)	30.20%	[1]	29.80%	[1]
Current Subordination (as a percent)	50.80%	[1]	52.60%	[1]
Prime first lien
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	278		333
Original Subordination (as a percent)	10.90%	[1]	10.90%	[1]
Current Subordination (as a percent)	3.20%	[1]	5.20%	[1]
Closed end second lien and HELOCs
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	25		49
Original Subordination (as a percent)			0.00%	[1]
CMBS
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	3,781		4,094
Original Subordination (as a percent)	33.30%	[1]	33.30%	[1]
Current Subordination (as a percent)	42.00%	[1]	41.80%	[1]
Other
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	8,913		9,310
Pooled Infrastructure Transactions [Member]
Net Par Outstanding on Credit Derivatives
Number of transactions	3
Other Transactions [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	5,700
Financial Guaranty Contracts Accounted For As Credit Derivatives Outstanding Principal Amount Rated BIG	$ 0.5

[1] Represents the sum of subordinate tranches and over-collateralization and does not include any benefit from excess interest collections that may be used to absorb losses.