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, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Total premiums earned	$ 85	$ 162	$ 202
United States [Member]
Segment Reporting Information [Line Items]
Total premiums earned	63	94	169
Other Americas [Member]
Segment Reporting Information [Line Items]
Total premiums earned	17	68	25
Other [Member]
Segment Reporting Information [Line Items]
Total premiums earned	$ 5	$ 0	$ 8