Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Segment Reporting Information [Line Items]
Total premiums earned	$ 77	$ 90	$ 228	$ 288
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	59	71	176	232
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	6	8	20	23
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	2	4	4
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	3	1	3	3
Other Americas [Member]
Segment Reporting Information [Line Items]
Total premiums earned	6	7	20	20
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	0	2	2
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 1	$ 1	$ 3	$ 4