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, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Segment Reporting Information [Line Items]
Total premiums earned	$ 53	$ 77	$ 146	$ 228
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	46	59	122	176
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	0	6	1	20
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	0	1	1	4
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	0	3	1	3
Other Americas [Member]
Segment Reporting Information [Line Items]
Total premiums earned	7	6	19	20
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	0	1	0	2
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 0	$ 1	$ 2	$ 3