Financial Guaranty Contracts Accounted for as Credit Derivatives - Credit Derivatives Subordination and Ratings (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2014		Dec. 31, 2013
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	$ 34,996		$ 54,482
Pooled corporate obligations [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	23,621		37,321
Original Subordination (as a percent)	30.10%	[1]	31.50%	[1]
Current Subordination (as a percent)	30.70%	[1]	30.60%	[1]
Pooled corporate obligations [Member] | Collateralized loan obligations and collateral bond obligations [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	11,688		19,323
Original Subordination (as a percent)	32.00%	[1]	32.40%	[1]
Current Subordination (as a percent)	36.90%	[1]	34.00%	[1]
Pooled corporate obligations [Member] | Synthetic investment grade pooled corporate [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	7,640		9,754
Original Subordination (as a percent)	22.60%	[1]	21.60%	[1]
Current Subordination (as a percent)	20.60%	[1]	20.00%	[1]
Pooled corporate obligations [Member] | Synthetic high yield pooled corporate [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	0		2,690
Original Subordination (as a percent)	0.00%	[1]	47.20%	[1]
Current Subordination (as a percent)	0.00%	[1]	41.10%	[1]
Pooled corporate obligations [Member] | TruPS CDOs [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	3,119		3,554
Original Subordination (as a percent)	45.30%	[1]	45.50%	[1]
Current Subordination (as a percent)	35.80%	[1]	32.90%	[1]
Pooled corporate obligations [Member] | Market Value of CDOs of corporate obligations [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	1,174		2,000
Original Subordination (as a percent)	19.10%	[1]	24.40%	[1]
Current Subordination (as a percent)	20.70%	[1]	30.50%	[1]
RMBS [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	2,986		5,826
Original Subordination (as a percent)	24.80%	[1]	24.40%	[1]
Current Subordination (as a percent)	33.90%	[1]	30.10%	[1]
RMBS [Member] | Option Adjustable Rate Mortgage and Alt-A Mortgage [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	1,378		2,609
Original Subordination (as a percent)	16.30%	[1]	19.20%	[1]
Current Subordination (as a percent)	10.70%	[1]	8.60%	[1]
RMBS [Member] | Subprime [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	1,366		2,930
Original Subordination (as a percent)	31.10%	[1]	30.50%	[1]
Current Subordination (as a percent)	50.50%	[1]	51.90%	[1]
RMBS [Member] | Prime [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	223		264
Original Subordination (as a percent)	10.90%	[1]	10.90%	[1]
Current Subordination (as a percent)	0.00%	[1]	3.20%	[1]
RMBS [Member] | Closed-end [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	19		23
Original Subordination (as a percent)	0.00%	[1]	0.00%	[1]
Current Subordination (as a percent)	0.00%	[1]	0.00%	[1]
CMBS [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	1,952		3,744
Original Subordination (as a percent)	35.30%	[1]	33.50%	[1]
Current Subordination (as a percent)	43.60%	[1]	42.50%	[1]
Other [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	$ 6,437		$ 7,591
Original Subordination (as a percent)	0.00%	[1]	0.00%	[1]
Current Subordination (as a percent)	0.00%	[1]	0.00%	[1]

[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.