Outstanding Exposure (Details 3) (USD $) In Millions, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Financial Guaranty Portfolio
Net Par Amount Outstanding	$ 567,098.0	$ 558,048.0
Net Economic Exposure to Selected European Countries	5,132
Mortgage guaranty insurance
Financial Guaranty Portfolio
Net Par Amount Outstanding	140.1
Italy
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	2,407
Greece
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	324
Portugal
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	246
Hungary
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	703
Spain
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	1,023
Ireland
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	429
Ireland | Mortgage guaranty insurance
Financial Guaranty Portfolio
Net Par Amount Outstanding	176.7
U.S. Public finance
Financial Guaranty Portfolio
Net Par Amount Outstanding	416,499.0	403,073.0
Structured finance
Financial Guaranty Portfolio
Net Par Amount Outstanding	87,784.0	92,234.0
Sovereign and sub-sovereign exposure
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	2,811
Sovereign and sub-sovereign exposure | Public finance
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	1,714
Sovereign and sub-sovereign exposure | Infrastructure finance
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	1,097
Sovereign and sub-sovereign exposure | Italy
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	1,381
Sovereign and sub-sovereign exposure | Italy | Public finance
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	1,040
Sovereign and sub-sovereign exposure | Italy | Infrastructure finance
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	341
Sovereign and sub-sovereign exposure | Greece
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	291
Sovereign and sub-sovereign exposure | Greece | Public finance
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	291
Sovereign and sub-sovereign exposure | Portugal
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	217
Sovereign and sub-sovereign exposure | Portugal | Public finance
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	113
Sovereign and sub-sovereign exposure | Portugal | Infrastructure finance
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	104
Sovereign and sub-sovereign exposure | Hungary
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	453
Sovereign and sub-sovereign exposure | Hungary | Infrastructure finance
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	453
Sovereign and sub-sovereign exposure | Spain
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	444
Sovereign and sub-sovereign exposure | Spain | Public finance
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	270
Sovereign and sub-sovereign exposure | Spain | Infrastructure finance
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	174
Sovereign and sub-sovereign exposure | Ireland
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	25
Sovereign and sub-sovereign exposure | Ireland | Infrastructure finance
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	25
Non-sovereign
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	2,321
Non-sovereign | Regulated utilities
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	243
Non-sovereign | RMBS
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	911
Non-sovereign | Commercial receivables
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	81
Non-sovereign | Pooled corporate obligations
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	1,086
Non-sovereign | Italy
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	1,026
Non-sovereign | Italy | Regulated utilities
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	226
Non-sovereign | Italy | RMBS
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	522
Non-sovereign | Italy | Commercial receivables
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	27
Non-sovereign | Italy | Pooled corporate obligations
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	251
Non-sovereign | Greece
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	33
Non-sovereign | Greece | Pooled corporate obligations
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	33
Non-sovereign | Portugal
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	29
Non-sovereign | Portugal | Commercial receivables
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	15
Non-sovereign | Portugal | Pooled corporate obligations
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	14
Non-sovereign | Hungary
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	250
Non-sovereign | Hungary | RMBS
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	249
Non-sovereign | Hungary | Commercial receivables
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	1
Non-sovereign | Spain
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	579
Non-sovereign | Spain | Regulated utilities
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	17
Non-sovereign | Spain | Commercial receivables
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	18
Non-sovereign | Spain | Pooled corporate obligations
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	544
Non-sovereign | Ireland
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	404
Non-sovereign | Ireland | RMBS
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	140
Non-sovereign | Ireland | Commercial receivables
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	20
Non-sovereign | Ireland | Pooled corporate obligations
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	244
BIG
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	1,373
BIG | Italy
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	252
BIG | Greece
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	291
BIG | Portugal
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	130
BIG | Hungary
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	540
BIG | Spain
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	145
BIG | Ireland
Financial Guaranty Portfolio
Net Economic Exposure to Selected European Countries	$ 15