Financial Guaranty Contracts Accounted for as Credit Derivatives - Credit Derivatives Subordination and Ratings (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015		Dec. 31, 2014
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	$ 31,731		$ 34,996
Pooled corporate obligations [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	20,735		23,621
Original Subordination (as a percent)	30.20%	[1]	30.10%	[1]
Current Subordination (as a percent)	31.20%	[1]	30.70%	[1]
Pooled corporate obligations [Member] | Collateralized loan obligations and collateral bond obligations [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	10,372		11,688
Original Subordination (as a percent)	32.00%	[1]	32.00%	[1]
Current Subordination (as a percent)	37.70%	[1]	36.90%	[1]
Pooled corporate obligations [Member] | Synthetic investment grade pooled corporate [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	6,363		7,640
Original Subordination (as a percent)	22.30%	[1]	22.60%	[1]
Current Subordination (as a percent)	20.20%	[1]	20.60%	[1]
Pooled corporate obligations [Member] | TruPS CDOs [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	2,948		3,119
Original Subordination (as a percent)	45.60%	[1]	45.30%	[1]
Current Subordination (as a percent)	37.50%	[1]	35.80%	[1]
Pooled corporate obligations [Member] | Market Value of CDOs of corporate obligations [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	1,052		1,174
Original Subordination (as a percent)	17.00%	[1]	19.10%	[1]
Current Subordination (as a percent)	15.70%	[1]	20.70%	[1]
RMBS [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	2,868		2,986
Original Subordination (as a percent)	24.80%	[1]	24.80%	[1]
Current Subordination (as a percent)	33.30%	[1]	33.90%	[1]
RMBS [Member] | Option Adjustable Rate Mortgage and Alt-A Mortgage [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	1,321		1,378
Original Subordination (as a percent)	16.20%	[1]	16.30%	[1]
Current Subordination (as a percent)	10.50%	[1]	10.70%	[1]
RMBS [Member] | Subprime [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	1,315		1,366
Original Subordination (as a percent)	31.30%	[1]	31.10%	[1]
Current Subordination (as a percent)	49.90%	[1]	50.50%	[1]
RMBS [Member] | Prime [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	213		223
Original Subordination (as a percent)	10.90%	[1]	10.90%	[1]
Current Subordination (as a percent)	0.00%	[1]	0.00%	[1]
RMBS [Member] | Closed-end [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	19		19
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	1,785		1,952
Original Subordination (as a percent)	35.90%	[1]	35.30%	[1]
Current Subordination (as a percent)	45.50%	[1]	43.60%	[1]
Other [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding	$ 6,343		$ 6,437
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.