Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Details) (USD $) In Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2011	Sep. 30, 2010	Sep. 30, 2011	Sep. 30, 2010
Segment Reporting Information [Line Items]
Total premiums earned	$ 199	$ 169	$ 544	$ 537
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	161	133	413	416
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	10	5	27	26
Europe (Excluding United Kingdom) [Member]
Segment Reporting Information [Line Items]
Total premiums earned	4	4	25	17
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	9	11	27	29
Central And South America [Member]
Segment Reporting Information [Line Items]
Total premiums earned	9	10	28	30
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	2	4	14	11
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 4	$ 2	$ 10	$ 8