Outstanding Exposure (Details 4) (USD $) In Millions, unless otherwise specified	Sep. 30, 2012		Dec. 31, 2011
Outstanding Exposure
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 Outstanding	$ 536,393		$ 558,048
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	74,684		85,047
BIG 1
Outstanding Exposure
Net Par Outstanding	9,155		12,250
BIG 2
Outstanding Exposure
Net Par Outstanding	5,875		4,981
BIG 3
Outstanding Exposure
Net Par Outstanding	9,339		9,531
Below investment grade (BIG)
Outstanding Exposure
Net Par Outstanding	24,369		26,762
% of Total Net Par Outstanding	4.50%		4.80%
Financial Guaranty Contracts Accounted for as Insurance Outstanding Principal Amount	17,475	[1]	18,959	[1]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	6,894		7,803
Financial Guaranty Contracts Accounted for as Insurance Number of Contracts	374	[1],[2]	368	[1],[2]
Financial Guaranty Contracts Accounted for as Credit Derivatives Number of Contracts	92	[2]	99	[2]
Insured Financial Obligations Number of Contracts	466	[2]	467	[2]
Below investment grade (BIG) | BIG 1
Outstanding Exposure
Net Par Outstanding	9,155		12,250
Financial Guaranty Contracts Accounted for as Insurance Outstanding Principal Amount	6,943	[1]	8,297	[1]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	2,212		3,953
Financial Guaranty Contracts Accounted for as Insurance Number of Contracts	148	[1],[2]	171	[1],[2]
Financial Guaranty Contracts Accounted for as Credit Derivatives Number of Contracts	31		40	[2]
Insured Financial Obligations Number of Contracts	179	[2]	211	[2]
Below investment grade (BIG) | BIG 2
Outstanding Exposure
Net Par Outstanding	5,875		4,981
Financial Guaranty Contracts Accounted for as Insurance Outstanding Principal Amount	3,233	[1]	3,458	[1]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	2,642		1,523
Financial Guaranty Contracts Accounted for as Insurance Number of Contracts	90	[1],[2]	71	[1],[2]
Financial Guaranty Contracts Accounted for as Credit Derivatives Number of Contracts	30		33	[2]
Insured Financial Obligations Number of Contracts	120	[2]	104	[2]
Below investment grade (BIG) | BIG 3
Outstanding Exposure
Net Par Outstanding	9,339		9,531
Financial Guaranty Contracts Accounted for as Insurance Outstanding Principal Amount	7,299	[1]	7,204	[1]
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	2,040		2,327
Financial Guaranty Contracts Accounted for as Insurance Number of Contracts	136	[1],[2]	126	[1],[2]
Financial Guaranty Contracts Accounted for as Credit Derivatives Number of Contracts	31		26	[2]
Insured Financial Obligations Number of Contracts	167	[2]	152	[2]
RMBS
Outstanding Exposure
Minimum Amount Assumed Principal Amount for Internal Rating	1
Residential, Prime, Financing Receivable [Member]
Outstanding Exposure
Net Par Outstanding	663		739
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	348		398
Residential, Prime, Financing Receivable [Member] | BIG 1
Outstanding Exposure
Net Par Outstanding	27		77
Residential, Prime, Financing Receivable [Member] | BIG 2
Outstanding Exposure
Net Par Outstanding	461		465
Residential, Prime, Financing Receivable [Member] | BIG 3
Outstanding Exposure
Net Par Outstanding	3		0
Residential, Prime, Financing Receivable [Member] | Below investment grade (BIG)
Outstanding Exposure
Net Par Outstanding	491		542
% of Total Net Par Outstanding	0.10%		0.10%
Alt-A First Mortgage [Member]
Outstanding Exposure
Net Par Outstanding	4,760		5,329
Alt-A First Mortgage [Member] | BIG 1
Outstanding Exposure
Net Par Outstanding	120		1,695
Alt-A First Mortgage [Member] | BIG 2
Outstanding Exposure
Net Par Outstanding	2,146		1,028
Alt-A First Mortgage [Member] | BIG 3
Outstanding Exposure
Net Par Outstanding	1,442		1,540
Alt-A First Mortgage [Member] | Below investment grade (BIG)
Outstanding Exposure
Net Par Outstanding	3,708		4,263
% of Total Net Par Outstanding	0.70%		0.80%
First Mortgage Option Adjustable Rate Mortgage [Member]
Outstanding Exposure
Net Par Outstanding	1,753		2,433
First Mortgage Option Adjustable Rate Mortgage [Member] | BIG 1
Outstanding Exposure
Net Par Outstanding	60		25
First Mortgage Option Adjustable Rate Mortgage [Member] | BIG 2
Outstanding Exposure
Net Par Outstanding	425		689
First Mortgage Option Adjustable Rate Mortgage [Member] | BIG 3
Outstanding Exposure
Net Par Outstanding	734		882
First Mortgage Option Adjustable Rate Mortgage [Member] | Below investment grade (BIG)
Outstanding Exposure
Net Par Outstanding	1,219		1,596
% of Total Net Par Outstanding	0.20%		0.30%
Residential, Subprime, Financing Receivable [Member]
Outstanding Exposure
Net Par Outstanding	7,541		8,136
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	3,634		4,012
Residential, Subprime, Financing Receivable [Member] | BIG 1
Outstanding Exposure
Net Par Outstanding	116		795
Residential, Subprime, Financing Receivable [Member] | BIG 2
Outstanding Exposure
Net Par Outstanding	1,350		1,200
Residential, Subprime, Financing Receivable [Member] | BIG 3
Outstanding Exposure
Net Par Outstanding	854		513
Residential, Subprime, Financing Receivable [Member] | Below investment grade (BIG)
Outstanding Exposure
Net Par Outstanding	2,320		2,508
% of Total Net Par Outstanding	0.40%		0.40%
Close Ended Second Mortgage [Member]
Outstanding Exposure
Net Par Outstanding	985		1,040
Close Ended Second Mortgage [Member] | BIG 1
Outstanding Exposure
Net Par Outstanding	0		0
Close Ended Second Mortgage [Member] | BIG 2
Outstanding Exposure
Net Par Outstanding	451		495
Close Ended Second Mortgage [Member] | BIG 3
Outstanding Exposure
Net Par Outstanding	412		520
Close Ended Second Mortgage [Member] | Below investment grade (BIG)
Outstanding Exposure
Net Par Outstanding	863		1,015
% of Total Net Par Outstanding	0.20%		0.20%
Home Equity Line of Credit [Member]
Outstanding Exposure
Net Par Outstanding	3,358		3,890
Home Equity Line of Credit [Member] | BIG 1
Outstanding Exposure
Net Par Outstanding	96		421
Home Equity Line of Credit [Member] | BIG 2
Outstanding Exposure
Net Par Outstanding	0		0
Home Equity Line of Credit [Member] | BIG 3
Outstanding Exposure
Net Par Outstanding	2,755		2,858
Home Equity Line of Credit [Member] | Below investment grade (BIG)
Outstanding Exposure
Net Par Outstanding	2,851		3,279
% of Total Net Par Outstanding	0.50%		0.60%
US Residential Mortgage Backed Securities [Member]
Outstanding Exposure
Net Par Outstanding	19,060		21,567
Financial Guaranty Contracts Accounted for as Credit Derivatives Outstanding Principal Amount	7,576		8,532
US Residential Mortgage Backed Securities [Member] | BIG 1
Outstanding Exposure
Net Par Outstanding	419		3,013
US Residential Mortgage Backed Securities [Member] | BIG 2
Outstanding Exposure
Net Par Outstanding	4,833		3,877
US Residential Mortgage Backed Securities [Member] | BIG 3
Outstanding Exposure
Net Par Outstanding	6,200		6,313
US Residential Mortgage Backed Securities [Member] | Below investment grade (BIG)
Outstanding Exposure
Net Par Outstanding	11,452		13,203
% of Total Net Par Outstanding	2.10%		2.40%
TruPS [Member]
Outstanding Exposure
Net Par Outstanding	5,863		6,334
TruPS [Member] | BIG 1
Outstanding Exposure
Net Par Outstanding	2,026		2,501
TruPS [Member] | BIG 2
Outstanding Exposure
Net Par Outstanding	0		0
TruPS [Member] | BIG 3
Outstanding Exposure
Net Par Outstanding	951		951
TruPS [Member] | Below investment grade (BIG)
Outstanding Exposure
Net Par Outstanding	2,977		3,452
% of Total Net Par Outstanding	0.60%		0.60%
Other Structured Finance [Member]
Outstanding Exposure
Net Par Outstanding	73,574		88,028
Other Structured Finance [Member] | BIG 1
Outstanding Exposure
Net Par Outstanding	1,156		1,295
Other Structured Finance [Member] | BIG 2
Outstanding Exposure
Net Par Outstanding	377		548
Other Structured Finance [Member] | BIG 3
Outstanding Exposure
Net Par Outstanding	1,370		1,429
Other Structured Finance [Member] | Below investment grade (BIG)
Outstanding Exposure
Net Par Outstanding	2,903		3,272
% of Total Net Par Outstanding	0.50%		0.60%
US Public Finance [Member]
Outstanding Exposure
Net Par Outstanding	399,176		403,073
US Public Finance [Member] | BIG 1
Outstanding Exposure
Net Par Outstanding	3,288		3,395
US Public Finance [Member] | BIG 2
Outstanding Exposure
Net Par Outstanding	665		274
US Public Finance [Member] | BIG 3
Outstanding Exposure
Net Par Outstanding	818		838
US Public Finance [Member] | Below investment grade (BIG)
Outstanding Exposure
Net Par Outstanding	4,771		4,507
% of Total Net Par Outstanding	0.90%		0.80%
Public finance Non-U.S.
Outstanding Exposure
Net Par Outstanding	38,720	[3]	39,046	[3]
Public finance Non-U.S. | Greece
Outstanding Exposure
Net Par Outstanding			282
Public finance Non-U.S. | BIG 1
Outstanding Exposure
Net Par Outstanding	2,266	[3]	2,046	[3]
Public finance Non-U.S. | BIG 2
Outstanding Exposure
Net Par Outstanding	0	[3]	282	[3]
Public finance Non-U.S. | BIG 3
Outstanding Exposure
Net Par Outstanding	0	[3]	0	[3]
Public finance Non-U.S. | Below investment grade (BIG)
Outstanding Exposure
Net Par Outstanding	$ 2,266	[3]	$ 2,328	[3]
% of Total Net Par Outstanding	0.40%	[3]	0.40%	[3]

[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.

[3] Includes $282 million in net par as of December 31, 2011, for bonds of the Hellenic Republic of Greece. See Note 4, Financial Guaranty Insurance Contracts.