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, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Segment Reporting Information [Line Items]
Total premiums earned	$ 134	$ 186	$ 265	$ 338
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	109	142	215	261
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	9	10	17	18
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	3	4	6	9
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	3	5	6	10
Central And South America [Member]
Segment Reporting Information [Line Items]
Total premiums earned	7	21	15	32
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	1	2	2
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 2	$ 3	$ 4	$ 6