Financial Guaranty Contracts Accounted for as Credit Derivatives - Credit Derivatives Subordination and Ratings (Details) - USD ($) $ in Millions		Mar. 31, 2016	Dec. 31, 2015
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 24,866	$ 25,594
Pooled corporate obligations [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 16,831	$ 17,523
Original Subordination (as a percent)	[1]	29.00%	29.20%
Current Subordination (as a percent)	[1]	31.90%	32.30%
Pooled corporate obligations [Member] | Collateralized loan obligations and collateral bond obligations [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 5,197	$ 5,873
Original Subordination (as a percent)	[1]	30.70%	30.90%
Current Subordination (as a percent)	[1]	42.80%	42.30%
Pooled corporate obligations [Member] | Synthetic investment grade pooled corporate [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 7,127	$ 7,108
Original Subordination (as a percent)	[1]	21.70%	21.70%
Current Subordination (as a percent)	[1]	19.40%	19.40%
Pooled corporate obligations [Member] | TruPS CDOs [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 3,394	$ 3,429
Original Subordination (as a percent)	[1]	45.60%	45.80%
Current Subordination (as a percent)	[1]	42.90%	42.60%
Pooled corporate obligations [Member] | Market Value of CDOs of corporate obligations [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 1,113	$ 1,113
Original Subordination (as a percent)	[1]	17.00%	17.00%
Current Subordination (as a percent)	[1]	27.80%	30.10%
RMBS [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 1,472	$ 1,526
Original Subordination (as a percent)	[1]	24.10%	24.10%
Current Subordination (as a percent)	[1]	37.30%	37.40%
RMBS [Member] | Option Adjustable Rate Mortgage and Alt-A Mortgage [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 336	$ 351
Original Subordination (as a percent)	[1]	10.50%	10.50%
Current Subordination (as a percent)	[1]	12.80%	12.70%
RMBS [Member] | Subprime [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 951	$ 981
Original Subordination (as a percent)	[1]	27.70%	27.70%
Current Subordination (as a percent)	[1]	45.00%	45.20%
RMBS [Member] | Prime [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 169	$ 177
Original Subordination (as a percent)	[1]	10.90%	10.90%
Current Subordination (as a percent)	[1]	0.00%	0.00%
RMBS [Member] | Closed-end [Member] | United States [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 16	$ 17
Original Subordination (as a percent)	[1]	0.00%	0.00%
Current Subordination (as a percent)	[1]	0.00%	0.00%
CMBS [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 496	$ 530
Original Subordination (as a percent)	[1]	44.70%	44.80%
Current Subordination (as a percent)	[1]	53.80%	52.60%
Other [Member]
Net Par Outstanding on Credit Derivatives
Net Par Outstanding		$ 6,067	$ 6,015
Original Subordination (as a percent)	[1]	0.00%	0.00%
Current Subordination (as a percent)	[1]	0.00%	0.00%

[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.