Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Details) (USD $) In Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Total premiums earned	$ 176	$ 182	$ 345	$ 368
United States [Member]
Total premiums earned	130	141	252	283
United Kingdom [Member]
Total premiums earned	10	8	17	21
Europe (Excluding United Kingdom) [Member]
Total premiums earned	9	7	21	12
Internationally Diversified [Member]
Total premiums earned	9	9	18	18
Central And South America [Member]
Total premiums earned	10	11	19	21
Asia [Member]
Total premiums earned	5	3	12	7
Other [Member]
Total premiums earned	$ 3	$ 3	$ 6	$ 6