Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Segment Reporting Information [Line Items]
Total premiums earned	$ 43	$ 75	$ 93	$ 151
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	35	58	76	117
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	7	1	14
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	0	2	1	3
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	0	1	0
Other Americas [Member]
Segment Reporting Information [Line Items]
Total premiums earned	5	7	12	14
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	0	0	0	1
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 1	$ 1	$ 2	$ 2