Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Segment Reporting Information [Line Items]
Total premiums earned	$ 708	$ 714	$ 868
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	545	555	661
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	37	35	29
Europe (Excluding United Kingdom) [Member]
Segment Reporting Information [Line Items]
Total premiums earned	29	23	21
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	33	41	83
Central And South America [Member]
Segment Reporting Information [Line Items]
Total premiums earned	36	40	41
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	15	9	17
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 13	$ 11	$ 16