Outstanding Exposure (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Financial guarantee obligations
Insured Financial Obligations Outstanding Principal Amount	$ 501,817	$ 519,893
Mortgage Guaranty Insured Obligations Outstanding Principal Amount	149
Gross Debt Service Outstanding	801,747	834,950
Net Debt Service Outstanding	751,741	782,180
US Public Finance
Financial guarantee obligations
Insured Financial Obligations Outstanding Principal Amount	378,456	387,967
Public finance Non-U.S.
Financial guarantee obligations
Insured Financial Obligations Outstanding Principal Amount	35,067	37,540
Public finance
Financial guarantee obligations
Gross Debt Service Outstanding	696,771	722,562
Net Debt Service Outstanding	654,113	677,369
Structured finance
Financial guarantee obligations
Gross Debt Service Outstanding	104,976	112,388
Net Debt Service Outstanding	97,628	104,811
Public finance Ireland
Financial guarantee obligations
Mortgage Guaranty Insured Obligations Outstanding Principal Amount	135
US Structured Finance [Member]
Financial guarantee obligations
Insured Financial Obligations Outstanding Principal Amount	70,280	74,695
Structured finance Non-U.S.
Financial guarantee obligations
Insured Financial Obligations Outstanding Principal Amount	$ 18,014	$ 19,691