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, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total premiums earned	$ 162	$ 202	$ 304
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	94	169	238
United Kingdom [Member]
Segment Reporting Information [Line Items]
Total premiums earned	0	2	27
Other Americas [Member]
Segment Reporting Information [Line Items]
Total premiums earned	68	25	26
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 0	$ 6	$ 13