Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Segment Reporting Information [Line Items]
Total premiums earned	$ 304	$ 385	$ 416
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	238	310	299
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	27	30	36
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	4	4	7
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	3	3	9
Other Americas [Member]
Segment Reporting Information [Line Items]
Total premiums earned	26	29	49
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	2	3	4
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 4	$ 6	$ 12