Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Segment Reporting Information [Line Items]
Total premiums earned	$ 168	$ 199	$ 506	$ 544
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	141	161	402	413
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	9	10	27	27
Europe (Excluding United Kingdom) [Member]
Segment Reporting Information [Line Items]
Total premiums earned	3	4	12	25
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	3	9	13	27
Central And South America [Member]
Segment Reporting Information [Line Items]
Total premiums earned	8	9	40	28
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	2	2	4	14
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 2	$ 4	$ 8	$ 10