Outstanding Exposure (Details 4) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013		Dec. 31, 2012
Financial guarantee obligations
Minimum Amount Assumed Principal Amount for Internal Rating	$ 1
Probability of paying more claims than being reimbursed (as a percent)	50.00%
Constant discount rate (as a percent)	5.00%
Maximum period of liquidity claims (in years)	1 year
Net Par Amount Outstanding	501,817		519,893
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	65,900		70,781
Category 1
Financial guarantee obligations
Net Par Amount Outstanding	7,879		9,254
Category 2
Financial guarantee obligations
Net Par Amount Outstanding	5,913		5,107
Category 3
Financial guarantee obligations
Net Par Amount Outstanding	8,562		9,031
Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	22,354		23,392
% of Total Net Par Outstanding	4.50%		4.50%
Financial Guaranty Contracts Accounted for as Insurance Outstanding Principal Amount	16,048	[1]	16,683	[1]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	6,306		6,709
Financial Guaranty Contracts Accounted for as Insurance Number of Contracts	372	[1],[2]	371	[1],[2]
Financial Guaranty Contracts Accounted for as Credit Derivatives Number of Contracts	85	[2]	89	[2]
Insured Financial Obligations Number of Contracts	457	[2]	460	[2]
Below investment grade (BIG) | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	7,879		9,254
Financial Guaranty Contracts Accounted for as Insurance Outstanding Principal Amount	6,148	[1]	7,049	[1]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	1,731		2,205
Financial Guaranty Contracts Accounted for as Insurance Number of Contracts	143	[1],[2]	153	[1],[2]
Financial Guaranty Contracts Accounted for as Credit Derivatives Number of Contracts	30		30	[2]
Insured Financial Obligations Number of Contracts	173	[2]	183	[2]
Below investment grade (BIG) | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	5,913		5,107
Financial Guaranty Contracts Accounted for as Insurance Outstanding Principal Amount	3,289	[1]	2,606	[1]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	2,624		2,501
Financial Guaranty Contracts Accounted for as Insurance Number of Contracts	82	[1],[2]	76	[1],[2]
Financial Guaranty Contracts Accounted for as Credit Derivatives Number of Contracts	26		27	[2]
Insured Financial Obligations Number of Contracts	108	[2]	103	[2]
Below investment grade (BIG) | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	8,562		9,031
Financial Guaranty Contracts Accounted for as Insurance Outstanding Principal Amount	6,611	[1]	7,028	[1]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	1,951		2,003
Financial Guaranty Contracts Accounted for as Insurance Number of Contracts	147	[1],[2]	142	[1],[2]
Financial Guaranty Contracts Accounted for as Credit Derivatives Number of Contracts	29		32	[2]
Insured Financial Obligations Number of Contracts	176	[2]	174	[2]
Prime first lien
Financial guarantee obligations
Net Par Amount Outstanding	619		641
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	318		333
Prime first lien | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	27		28
Prime first lien | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	422		436
Prime first lien | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	10		11
Prime first lien | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	459		475
% of Total Net Par Outstanding	0.10%		0.10%
Alt-A first lien
Financial guarantee obligations
Net Par Amount Outstanding	4,439		4,589
Alt-A first lien | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	98		109
Alt-A first lien | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	1,937		1,987
Alt-A first lien | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	1,430		1,479
Alt-A first lien | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	3,465		3,575
% of Total Net Par Outstanding	0.70%		0.70%
Option ARM
Financial guarantee obligations
Net Par Amount Outstanding	1,387		1,550
Option ARM | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	65		61
Option ARM | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	339		392
Option ARM | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	362		643
Option ARM | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	766		1,096
% of Total Net Par Outstanding	0.20%		0.20%
First mortgage, subprime
Financial guarantee obligations
Net Par Amount Outstanding	7,130		7,330
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	3,360		3,494
First mortgage, subprime | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	162		152
First mortgage, subprime | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	1,219		1,161
First mortgage, subprime | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	1,006		1,024
First mortgage, subprime | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	2,387		2,337
% of Total Net Par Outstanding	0.50%		0.40%
Close ended second lien
Financial guarantee obligations
Net Par Amount Outstanding	452		521
Close ended second lien | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	0		0
Close ended second lien | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	61		247
Close ended second lien | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	279		157
Close ended second lien | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	340		404
% of Total Net Par Outstanding	0.10%		0.10%
HELOCs
Financial guarantee obligations
Net Par Amount Outstanding	3,059		3,196
HELOCs | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	72		91
HELOCs | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	12		0
HELOCs | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	2,526		2,627
HELOCs | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	2,610		2,718
% of Total Net Par Outstanding	0.50%		0.50%
U.S. RMBS
Financial guarantee obligations
Net Par Amount Outstanding	17,086		17,827
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	6,984		7,257
U.S. RMBS | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	424		441
U.S. RMBS | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	3,990		4,223
U.S. RMBS | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	5,613		5,941
U.S. RMBS | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	10,027		10,605
% of Total Net Par Outstanding	2.10%		2.00%
TruPS
Financial guarantee obligations
Net Par Amount Outstanding	5,496		5,693
TruPS | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	1,873		1,920
TruPS | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	0		0
TruPS | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	941		952
TruPS | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	2,814		2,872
% of Total Net Par Outstanding	0.50%		0.60%
Other Structured Finance [Member]
Financial guarantee obligations
Net Par Amount Outstanding	65,712		70,866
Other Structured Finance [Member] | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	1,240		1,310
Other Structured Finance [Member] | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	379		384
Other Structured Finance [Member] | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	1,298		1,325
Other Structured Finance [Member] | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	2,917		3,019
% of Total Net Par Outstanding	0.60%		0.60%
US Public Finance
Financial guarantee obligations
Net Par Amount Outstanding	378,456		387,967
US Public Finance | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	3,359		3,290
US Public Finance | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	520		500
US Public Finance | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	710		813
US Public Finance | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	4,589		4,603
% of Total Net Par Outstanding	0.90%		0.90%
Public finance Non-U.S.
Financial guarantee obligations
Net Par Amount Outstanding	35,067		37,540
Public finance Non-U.S. | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	983		2,293
Public finance Non-U.S. | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	1,024		0
Public finance Non-U.S. | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	0		0
Public finance Non-U.S. | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	$ 2,007		$ 2,293
% of Total Net Par Outstanding	0.40%		0.40%

[1] Includes net par outstanding for FG VIEs.

[2] A risk represents the aggregate of the financial guaranty policies that share the same revenue source for purposes of making Debt Service payments.