Outstanding Exposure (Details 4) (USD $)	Mar. 31, 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	$ 567,098,000,000	$ 558,048,000,000
Net Par Outstanding Credit Derivatives	83,002,000,000	85,047,000,000
Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	25,999,000,000	26,762,000,000
% of Total Net Par Outstanding	4.60%	4.80%
Net Par Outstanding Financial Guaranty Insurance	18,519,000,000	18,959,000,000
Net Par Outstanding Credit Derivatives	7,479,000,000	8,061,000,000
Number of Risks in Financial Guaranty Insurance	368	368
Number of Risks in Credit Derivative	94	99
Number of Risks in Category	462	467
Below investment grade ("BIG") | BIG 1
Outstanding Exposure
Net Par Outstanding	10,740,000,000	12,250,000,000
Net Par Outstanding Financial Guaranty Insurance	7,703,000,000	8,297,000,000
Net Par Outstanding Credit Derivatives	3,037,000,000	3,953,000,000
Number of Risks in Financial Guaranty Insurance	164	171
Number of Risks in Credit Derivative	33	40
Number of Risks in Category	197	211
Below investment grade ("BIG") | BIG 2
Outstanding Exposure
Net Par Outstanding	6,116,000,000	4,981,000,000
Net Par Outstanding Financial Guaranty Insurance	3,903,000,000	3,458,000,000
Net Par Outstanding Credit Derivatives	2,213,000,000	1,523,000,000
Number of Risks in Financial Guaranty Insurance	79	71
Number of Risks in Credit Derivative	35	33
Number of Risks in Category	114	104
Below investment grade ("BIG") | BIG 3
Outstanding Exposure
Net Par Outstanding	9,143,000,000	9,531,000,000
Net Par Outstanding Financial Guaranty Insurance	6,913,000,000	7,204,000,000
Net Par Outstanding Credit Derivatives	2,230,000,000	2,327,000,000
Number of Risks in Financial Guaranty Insurance	125	126
Number of Risks in Credit Derivative	26	26
Number of Risks in Category	151	152
RMBS
Outstanding Exposure
Minimum par of assumed credits for internal modeling to determine credit rating	1,000,000
U.S. RMBS
Outstanding Exposure
Net Par Outstanding	20,873,000,000	21,567,000,000
Net Par Outstanding Credit Derivatives	8,319,000,000	8,532,000,000
U.S. RMBS | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	12,725,000,000	13,203,000,000
% of Total Net Par Outstanding	2.30%	2.40%
U.S. RMBS | Below investment grade ("BIG") | BIG 1
Outstanding Exposure
Net Par Outstanding	1,649,000,000	3,013,000,000
U.S. RMBS | Below investment grade ("BIG") | BIG 2
Outstanding Exposure
Net Par Outstanding	5,090,000,000	3,877,000,000
U.S. RMBS | Below investment grade ("BIG") | BIG 3
Outstanding Exposure
Net Par Outstanding	5,986,000,000	6,313,000,000
Prime first lien
Outstanding Exposure
Net Par Outstanding	713,000,000	739,000,000
Net Par Outstanding Credit Derivatives	382,000,000	398,000,000
Prime first lien | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	525,000,000	542,000,000
% of Total Net Par Outstanding	0.10%	0.10%
Prime first lien | Below investment grade ("BIG") | BIG 1
Outstanding Exposure
Net Par Outstanding	77,000,000	77,000,000
Prime first lien | Below investment grade ("BIG") | BIG 2
Outstanding Exposure
Net Par Outstanding	448,000,000	465,000,000
Alt-A first lien
Outstanding Exposure
Net Par Outstanding	5,208,000,000	5,329,000,000
Alt-A first lien | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	4,172,000,000	4,263,000,000
% of Total Net Par Outstanding	0.70%	0.80%
Alt-A first lien | Below investment grade ("BIG") | BIG 1
Outstanding Exposure
Net Par Outstanding	962,000,000	1,695,000,000
Alt-A first lien | Below investment grade ("BIG") | BIG 2
Outstanding Exposure
Net Par Outstanding	1,717,000,000	1,028,000,000
Alt-A first lien | Below investment grade ("BIG") | BIG 3
Outstanding Exposure
Net Par Outstanding	1,493,000,000	1,540,000,000
Option ARM
Outstanding Exposure
Net Par Outstanding	2,256,000,000	2,433,000,000
Option ARM | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	1,463,000,000	1,596,000,000
% of Total Net Par Outstanding	0.30%	0.30%
Option ARM | Below investment grade ("BIG") | BIG 1
Outstanding Exposure
Net Par Outstanding	1,000,000	25,000,000
Option ARM | Below investment grade ("BIG") | BIG 2
Outstanding Exposure
Net Par Outstanding	687,000,000	689,000,000
Option ARM | Below investment grade ("BIG") | BIG 3
Outstanding Exposure
Net Par Outstanding	775,000,000	882,000,000
Subprime
Outstanding Exposure
Net Par Outstanding	7,976,000,000	8,136,000,000
Net Par Outstanding Credit Derivatives	3,925,000,000	4,012,000,000
Subprime | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	2,451,000,000	2,508,000,000
% of Total Net Par Outstanding	0.40%	0.40%
Subprime | Below investment grade ("BIG") | BIG 1
Outstanding Exposure
Net Par Outstanding	208,000,000	795,000,000
Subprime | Below investment grade ("BIG") | BIG 2
Outstanding Exposure
Net Par Outstanding	1,745,000,000	1,200,000,000
Subprime | Below investment grade ("BIG") | BIG 3
Outstanding Exposure
Net Par Outstanding	498,000,000	513,000,000
Closed-end second lien
Outstanding Exposure
Net Par Outstanding	1,020,000,000	1,040,000,000
Closed-end second lien | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	997,000,000	1,015,000,000
% of Total Net Par Outstanding	0.20%	0.20%
Closed-end second lien | Below investment grade ("BIG") | BIG 2
Outstanding Exposure
Net Par Outstanding	493,000,000	495,000,000
Closed-end second lien | Below investment grade ("BIG") | BIG 3
Outstanding Exposure
Net Par Outstanding	504,000,000	520,000,000
Home equity lines of credit ("HELOC's")
Outstanding Exposure
Net Par Outstanding	3,700,000,000	3,890,000,000
Home equity lines of credit ("HELOC's") | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	3,117,000,000	3,279,000,000
% of Total Net Par Outstanding	0.60%	0.60%
Home equity lines of credit ("HELOC's") | Below investment grade ("BIG") | BIG 1
Outstanding Exposure
Net Par Outstanding	401,000,000	421,000,000
Home equity lines of credit ("HELOC's") | Below investment grade ("BIG") | BIG 3
Outstanding Exposure
Net Par Outstanding	2,716,000,000	2,858,000,000
Trust preferred securities ("TruPS")
Outstanding Exposure
Net Par Outstanding	6,272,000,000	6,334,000,000
Trust preferred securities ("TruPS") | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	3,092,000,000	3,452,000,000
% of Total Net Par Outstanding	0.50%	0.60%
Trust preferred securities ("TruPS") | Below investment grade ("BIG") | BIG 1
Outstanding Exposure
Net Par Outstanding	2,140,000,000	2,501,000,000
Trust preferred securities ("TruPS") | Below investment grade ("BIG") | BIG 3
Outstanding Exposure
Net Par Outstanding	952,000,000	951,000,000
Other structured finance
Outstanding Exposure
Net Par Outstanding	83,541,000,000	88,028,000,000
Other structured finance | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	3,260,000,000	3,272,000,000
% of Total Net Par Outstanding	0.60%	0.60%
Other structured finance | Below investment grade ("BIG") | BIG 1
Outstanding Exposure
Net Par Outstanding	1,410,000,000	1,295,000,000
Other structured finance | Below investment grade ("BIG") | BIG 2
Outstanding Exposure
Net Par Outstanding	465,000,000	548,000,000
Other structured finance | Below investment grade ("BIG") | BIG 3
Outstanding Exposure
Net Par Outstanding	1,385,000,000	1,429,000,000
U.S. Public finance
Outstanding Exposure
Net Par Outstanding	416,499,000,000	403,073,000,000
U.S. Public finance | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	4,570,000,000	4,507,000,000
% of Total Net Par Outstanding	0.80%	0.80%
U.S. Public finance | Below investment grade ("BIG") | BIG 1
Outstanding Exposure
Net Par Outstanding	3,480,000,000	3,395,000,000
U.S. Public finance | Below investment grade ("BIG") | BIG 2
Outstanding Exposure
Net Par Outstanding	270,000,000	274,000,000
U.S. Public finance | Below investment grade ("BIG") | BIG 3
Outstanding Exposure
Net Par Outstanding	820,000,000	838,000,000
Public finance Non-U.S.
Outstanding Exposure
Net Par Outstanding	39,913,000,000	39,046,000,000
Public finance Non-U.S. | Hellenic Republic of Greece
Outstanding Exposure
Net Par Outstanding	291,000,000	282,000,000
Net expected to be paid	231,900,000	42,600,000
Public finance Non-U.S. | Below investment grade ("BIG")
Outstanding Exposure
Net Par Outstanding	2,352,000,000	2,328,000,000
% of Total Net Par Outstanding	0.40%	0.40%
Public finance Non-U.S. | Below investment grade ("BIG") | BIG 1
Outstanding Exposure
Net Par Outstanding	2,061,000,000	2,046,000,000
Public finance Non-U.S. | Below investment grade ("BIG") | BIG 2
Outstanding Exposure
Net Par Outstanding	291,000,000	282,000,000
Public finance
Outstanding Exposure
Net Par Outstanding	456,412,000,000	442,119,000,000
Public finance | Below investment grade ("BIG")
Outstanding Exposure
% of Total Net Par Outstanding	1.20%	1.20%
Total first lien | Bank of America agreement
Outstanding Exposure
Loss sharing agreement (as a percent)	80.00%
Maximum aggregate collateral losses	6,600,000,000
Total first lien | Below investment grade ("BIG") | Bank of America agreement
Outstanding Exposure
Net Par Outstanding	$ 345,600,000
% of Total Net Par Outstanding	20.00%