Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Segment Reporting Information [Line Items]
Total premiums earned	$ 661	$ 708	$ 714
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	530	545	555
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	36	37	35
Europe (Excluding United Kingdom) [Member]
Segment Reporting Information [Line Items]
Total premiums earned	15	29	23
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	16	33	41
Central And South America [Member]
Segment Reporting Information [Line Items]
Total premiums earned	48	36	40
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	5	15	9
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 11	$ 13	$ 11