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		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Segment Reporting Information [Line Items]
Total premiums earned	$ 186	$ 176	$ 338	$ 345
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	142	130	261	252
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	10	10	18	17
Europe (Excluding United Kingdom) [Member]
Segment Reporting Information [Line Items]
Total premiums earned	4	9	9	21
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	5	9	10	18
Central And South America [Member]
Segment Reporting Information [Line Items]
Total premiums earned	21	10	32	19
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	5	2	12
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 3	$ 3	$ 6	$ 6