Outstanding Exposure (Details 3) (USD $)	9 Months Ended
	Sep. 30, 2013		Dec. 31, 2012
Financial guarantee obligations
Minimum Amount Assumed Principal Amount for Internal Rating	$ 1,000,000
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	473,370,000,000	[1]	518,772,000,000	[1]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	59,109,000,000		70,781,000,000
Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	18,557,000,000	[1]	22,297,000,000	[1]
% of Total Net Par Outstanding	3.90%		4.30%
Financial Guaranty Contracts Accounted for as Insurance Outstanding Principal Amount	13,578,000,000	[2]	15,588,000,000	[2]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	4,979,000,000		6,709,000,000
Financial Guaranty Contracts Accounted for as Insurance Number of Contracts	366	[2],[3]	370	[2],[3]
Financial Guaranty Contracts Accounted for as Credit Derivatives Number of Contracts	75	[3]	89	[3]
Insured Financial Obligations Number of Contracts	441	[3]	459	[3]
Category 1
Financial guarantee obligations
Net Par Amount Outstanding	8,986,000,000		10,820,000,000
Category 1 | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	8,986,000,000		10,820,000,000
Financial Guaranty Contracts Accounted for as Insurance Outstanding Principal Amount	7,453,000,000	[2]	7,929,000,000	[2]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	1,533,000,000		2,891,000,000
Financial Guaranty Contracts Accounted for as Insurance Number of Contracts	154	[2],[3]	163	[2],[3]
Financial Guaranty Contracts Accounted for as Credit Derivatives Number of Contracts	21	[3]	33	[3]
Insured Financial Obligations Number of Contracts	175	[3]	196	[3]
Category 2
Financial guarantee obligations
Net Par Amount Outstanding	4,805,000,000		4,617,000,000
Category 2 | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	4,805,000,000		4,617,000,000
Financial Guaranty Contracts Accounted for as Insurance Outstanding Principal Amount	2,537,000,000	[2]	2,116,000,000	[2]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	2,268,000,000		2,501,000,000
Financial Guaranty Contracts Accounted for as Insurance Number of Contracts	76	[2],[3]	76	[2],[3]
Financial Guaranty Contracts Accounted for as Credit Derivatives Number of Contracts	25	[3]	27	[3]
Insured Financial Obligations Number of Contracts	101	[3]	103	[3]
Category 3
Financial guarantee obligations
Net Par Amount Outstanding	4,766,000,000		6,860,000,000
Category 3 | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	4,766,000,000		6,860,000,000
Financial Guaranty Contracts Accounted for as Insurance Outstanding Principal Amount	3,588,000,000	[2]	5,543,000,000	[2]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	1,178,000,000		1,317,000,000
Financial Guaranty Contracts Accounted for as Insurance Number of Contracts	136	[2],[3]	131	[2],[3]
Financial Guaranty Contracts Accounted for as Credit Derivatives Number of Contracts	29	[3]	29	[3]
Insured Financial Obligations Number of Contracts	165	[3]	160	[3]
Prime first lien
Financial guarantee obligations
Net Par Amount Outstanding	561,000,000		641,000,000
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	278,000,000		333,000,000
Prime first lien | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	417,000,000		475,000,000
% of Total Net Par Outstanding	0.10%		0.10%
Prime first lien | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	75,000,000		28,000,000
Prime first lien | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	333,000,000		436,000,000
Prime first lien | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	9,000,000		11,000,000
Alt-A first lien
Financial guarantee obligations
Net Par Amount Outstanding	3,993,000,000		4,469,000,000
Alt-A first lien | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	2,801,000,000		3,454,000,000
% of Total Net Par Outstanding	0.60%		0.70%
Alt-A first lien | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	913,000,000		753,000,000
Alt-A first lien | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	1,454,000,000		1,962,000,000
Alt-A first lien | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	434,000,000		739,000,000
Option ARM
Financial guarantee obligations
Net Par Amount Outstanding	1,014,000,000		1,450,000,000
Option ARM | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	632,000,000		1,042,000,000
% of Total Net Par Outstanding	0.20%		0.20%
Option ARM | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	68,000,000		333,000,000
Option ARM | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	353,000,000		392,000,000
Option ARM | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	211,000,000		317,000,000
First mortgage, subprime
Financial guarantee obligations
Net Par Amount Outstanding	6,335,000,000		7,048,000,000
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	3,067,000,000		3,494,000,000
First mortgage, subprime | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	1,983,000,000		2,061,000,000
% of Total Net Par Outstanding	0.40%		0.40%
First mortgage, subprime | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	172,000,000		152,000,000
First mortgage, subprime | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	921,000,000		988,000,000
First mortgage, subprime | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	890,000,000		921,000,000
Close ended second lien
Financial guarantee obligations
Net Par Amount Outstanding	252,000,000		348,000,000
Close ended second lien | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	150,000,000		231,000,000
% of Total Net Par Outstanding	0.00%		0.00%
Close ended second lien | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	9,000,000		97,000,000
Close ended second lien | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	20,000,000		76,000,000
Close ended second lien | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	121,000,000		58,000,000
HELOCs
Financial guarantee obligations
Net Par Amount Outstanding	2,365,000,000		3,079,000,000
HELOCs | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	1,955,000,000		2,576,000,000
% of Total Net Par Outstanding	0.40%		0.50%
HELOCs | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	1,461,000,000		644,000,000
HELOCs | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	22,000,000		0
HELOCs | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	472,000,000		1,932,000,000
U.S. RMBS
Financial guarantee obligations
Net Par Amount Outstanding	14,520,000,000		17,035,000,000
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	6,365,000,000		7,257,000,000
U.S. RMBS | Investment Grade [Member]
Financial guarantee obligations
Refinement of Approach to Internal Credit Ratings, Number of Transactions Upgraded	4		1
Refinement of Approach to Internal Credit Ratings, Net Par Upgraded	264,000,000		25,000,000
U.S. RMBS | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	7,938,000,000		9,839,000,000
% of Total Net Par Outstanding	1.70%		1.90%
U.S. RMBS | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	2,698,000,000		2,007,000,000
U.S. RMBS | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	3,103,000,000		3,854,000,000
U.S. RMBS | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	2,137,000,000		3,978,000,000
TruPS
Financial guarantee obligations
Net Par Amount Outstanding	5,164,000,000		5,694,000,000
TruPS | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	1,996,000,000		2,873,000,000
% of Total Net Par Outstanding	0.40%		0.60%
TruPS | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	941,000,000		1,920,000,000
TruPS | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	136,000,000		0
TruPS | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	919,000,000		953,000,000
Other Structured Finance [Member]
Financial guarantee obligations
Net Par Amount Outstanding	57,571,000,000		70,574,000,000
Other Structured Finance [Member] | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	2,369,000,000		2,727,000,000
% of Total Net Par Outstanding	0.50%		0.50%
Other Structured Finance [Member] | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	1,192,000,000		1,310,000,000
Other Structured Finance [Member] | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	312,000,000		263,000,000
Other Structured Finance [Member] | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	865,000,000		1,154,000,000
US Public Finance
Financial guarantee obligations
Net Par Amount Outstanding	361,203,000,000	[1]	387,929,000,000	[1]
US Public Finance | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	4,628,000,000	[1]	4,565,000,000	[1]
% of Total Net Par Outstanding	1.00%		0.90%
US Public Finance | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	3,154,000,000		3,290,000,000
US Public Finance | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	629,000,000		500,000,000
US Public Finance | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	845,000,000		775,000,000
Public finance Non-U.S.
Financial guarantee obligations
Net Par Amount Outstanding	34,912,000,000	[1]	37,540,000,000	[1]
Public finance Non-U.S. | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	1,626,000,000	[1]	2,293,000,000	[1]
% of Total Net Par Outstanding	0.30%		0.40%
Public finance Non-U.S. | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	1,001,000,000		2,293,000,000
Public finance Non-U.S. | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	625,000,000		0
Public finance Non-U.S. | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	0		0
Previous Approach [Member] | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	18,821,000,000		22,322,000,000
Previous Approach [Member] | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	7,032,000,000		9,254,000,000
Previous Approach [Member] | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	4,805,000,000		4,617,000,000
Previous Approach [Member] | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	6,984,000,000		8,451,000,000
New Approach [Member] | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	18,557,000,000		22,297,000,000
New Approach [Member] | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	8,986,000,000		10,820,000,000
New Approach [Member] | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	4,805,000,000		4,617,000,000
New Approach [Member] | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	4,766,000,000		6,860,000,000
Difference [Member] | Below investment grade (BIG)
Financial guarantee obligations
Net Par Amount Outstanding	(264,000,000)		(25,000,000)
Difference [Member] | Category 1
Financial guarantee obligations
Net Par Amount Outstanding	1,954,000,000		1,566,000,000
Difference [Member] | Category 2
Financial guarantee obligations
Net Par Amount Outstanding	0		0
Difference [Member] | Category 3
Financial guarantee obligations
Net Par Amount Outstanding	$ (2,218,000,000)		$ (1,591,000,000)

[1] In Third Quarter 2013, the Company adjusted its approach to assigning internal ratings. See "Refinement of Approach to Internal Credit Ratings and Surveillance Categories" below. This approach is reflected in the "Financial Guaranty Portfolio by Internal Rating" tables as of both September 30, 2013 and December 31, 2012.

[2] Includes net par outstanding for FG VIEs.

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