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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Segment Reporting Information [Line Items]
Total premiums earned	$ 93	$ 93	$ 198	$ 186
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	74	71	161	138
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	8	10	15	19
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	0	1	2	4
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	1	2	3
Central And South America [Member]
Segment Reporting Information [Line Items]
Total premiums earned	7	7	13	16
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	1	2	2
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 2	$ 2	$ 3	$ 4