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, 2015	Dec. 31, 2014	Dec. 31, 2013
Segment Reporting Information [Line Items]
Total premiums earned	$ 385	$ 416	$ 494
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	310	299	391
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	30	36	34
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	4	7	11
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	3	9	9
Central And South America [Member]
Segment Reporting Information [Line Items]
Total premiums earned	29	49	36
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	3	4	4
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 6	$ 12	$ 9