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, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Segment Reporting Information [Line Items]
Total premiums earned	$ 75	$ 93	$ 151	$ 198
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	58	74	117	161
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	7	8	14	15
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	2	0	3	2
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	0	1	0	2
Other Americas [Member]
Segment Reporting Information [Line Items]
Total premiums earned	7	7	14	13
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	0	1	1	2
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 1	$ 2	$ 2	$ 3