Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Segment Reporting Information [Line Items]
Total premiums earned	$ 50	$ 76
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	41	59
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	0	7
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	1
Other Americas [Member]
Segment Reporting Information [Line Items]
Total premiums earned	7	7
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	0	1
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 1	$ 1