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
	Mar. 31, 2013	Mar. 31, 2012
Segment Reporting Information [Line Items]
Total premiums earned	$ 131	$ 152
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	106	119
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	8	8
Europe (Excluding United Kingdom) [Member]
Segment Reporting Information [Line Items]
Total premiums earned	3	5
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	3	5
Central And South America [Member]
Segment Reporting Information [Line Items]
Total premiums earned	8	11
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	1
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 2	$ 3