Business Segments (Summary Of Premiums Earned On Financial Guarantees And Insured Derivatives By Geographic Location Of Risk) (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Segment Reporting Information [Line Items]
Total premiums earned	$ 105	$ 93
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	87	67
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	7	9
Europe Excluding United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	2	3
Internationally Diversified [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	2
Central And South America [Member]
Segment Reporting Information [Line Items]
Total premiums earned	6	9
Asia [Member]
Segment Reporting Information [Line Items]
Total premiums earned	1	1
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 1	$ 2